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Seasonal changes in a sandy beach fish assemblage at Porto Pim, Faial, Azores
Estuarine, Coastal and Shelf Science (1995) 41, 579-591
Seasonal Changes in a Sandy B e a c h Fish A s s e m b l a g e at Porto P i m , Faial,-Azores
Ricardo S. Santos a and Richard D. M. Nash b ~Departamento de Oceanografia e Pescas, Universidade dos A~ores, PT-9900 Horta, Faial, Azores, Portugal and bPort Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man IM9 6JA, U.K. Received 19 August 1993 and in revised form 13 September 1994
Keywords: Azores; shallow water; fish; seasonal A shallow-water fish assemblage, over a soft, sandy substratum at Porto Pim Bay on the island of Faial, Azores, was sampled at monthly intervals with a beach seine. Sampling was undertaken at 3-h intervals over 24 h'on a monthly basis between July 1989 and June 1990. There was seasonal variation in the density of individuals and biomass with the highest values in summer (1 "4 m - 2 and 27-75 g m 2 respectively). A total of 24 species were observed and the assemblage was dominated by four species (Chelon labrosus, Sardina pilchardus, Pagellus bogaraveo, Trachinotus ovatus). Species diversity (H') was high throughout the year but equatability (_7') was low, due to the dominance of a few species. The largest change in the assemblage structure occurred between March and April. Only a few species could be classified as resident, with the two predators (Dasyatis pastinaca and Echiichthys vipera) being abundant at different times of the year. Most of the species observed were juvenile fish, many with an offshore distribution as adults, using Porto Pim Bay as a nursery ground. c~ 1995 Academic Press Limited
Introduction T h e r e have b e e n a n u m b e r of studies which have e x a m i n e d the seasonal variations in shallow-water fish assemblage structure o n soft sediments ( H o r n , 1980; Allen & D e M a r t i n i , 1983; Allen et al., 1983; Lasiak, 1984b; N a s h , 1988; Wright, 1988, 1989; Ali & H u s s a i n , 1990). T h e s e studies vary from descriptions of the fish assemblages a n d their d y n a m i c s (e.g. H o r n , 1980) to a n e x a m i n a t i o n of the effects of severe seasonal changes in e n v i r o n m e n t a l c o n d i t i o n s (Mash, 1988). Shallow-water, s a n d - b o t t o m areas in the Azores are relatively scarce a n d it is only recently that this habitat has b e e n studied (Nash et al., 1990, 1991, 1994a; N a s h & Santos, 1993). T h e r e is a fairly large range in the potential density of fish a n d s t a n d i n g crops in shallow-water areas (Ross et al., 1987), a n d it is u n k n o w n whether the relatively rare shallow-water habitat in the Azores is high or low with respect to these other temperate a n d subtropical areas. T h e Azores islands are also generally well separated a n d as a group they are approximately 1700 k m from the nearest c o n t i n e n t . D u e to the close proximity of very deep water, there is also the possibility of interactions b e t w e e n coastal a n d oceanic species as well as shallow-water a n d deep-water species. 0272-7714/95/050579+ 13 $12.00/0
© 1995 Academic Press Limited
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R. S. Santos & R. D. M. Nash
This study provides a description of the seasonal fluctuations in a shallow-water, soft-sediment fish assemblage, and c o m p a r e s this with o t h e r studies u n d e r t a k e n elsewhere b o t h in other subtropical and in t e m p e r a t e areas.
S t u d y site T h e Azores are a small group of islands (which cover a fairly large area) located along the m i d - A t l a n t i c ridge in the region of 38°N 28°W. T h e s a m p l i n g site, Porto Pim, is located on the south-east corner of the island of Faial (Figure 1). T h e beach is small (approximately 280 m in length) facing south-west. T h e n o r t h a n d south coasts of the beach area are rocky. T h e southern b o r d e r is the edge of the M o n t e da G u i a p r o t e c t e d area. T h e m a x i m u m tidal range here is 1 m. T h e m a x i m u m horizontal intertidal area of the beach at low water is a p p r o x i m a t e l y 30 m.
Methods Sampling was u n d e r t a k e n with a 20 m b e a c h seine (32 m m stretch m e s h in the wings reducing t h r o u g h 15 m m to 8 m m in the centre). T w o sets of the b e a c h seine were m a d e at 3-h intervals for 24 h on each s a m p l i n g date. S a m p l i n g c o m m e n c e d in July 1989 a n d finished in June 1990 at a p p r o x i m a t e l y 1 - m o n t h intervals. E x t r e m e l y b a d w e a t h e r in D e c e m b e r 1989 p r e v e n t e d s a m p l i n g on that occasion. T h e b e a c h seine was fitted with 25 m hauling ropes and set parallel to the beach. T h e net swept an area from a p p r o x i m a t e l y 4 m d e p t h to the shore, with each haul having an effective fishing area o f 291 m 2 [(using the m e t h o d o l o g y of K u b e c k a & B o h m (1991)]. N o estimates o f overall efficiency are available for this net on this beach. W a t e r t e m p e r a t u r e was o b t a i n e d from H o r t a H a r b o u r as part o f the general daily m o n i t o r i n g p r o g r a m m e c o n d u c t e d by the D e p a r t m e n t of O c e a n o g r a p h y a n d Fisheries, University of the Azores, Horta. T h e data given here are for the specific s a m p l i n g dates used. All species were identified using W h i t e h e a d et al. (1986). T h e total n u m b e r of individuals and total weight for each species in each haul was obtained. T h e lengths (to the nearest 1 m m ) a n d individual weights (to the nearest 1 g) (using a s u b s a m p l e of up to 100 individuals where necessary) were m e a s u r e d for each sample. In this report, all m o n t h l y catches are r e p o r t e d as a s u m m a t i o n of the two hauls taken at each s a m p l i n g time and the eight samples taken over the 24-h p e r i o d (i.e. 16 samples). Average catch (numbers and biomass) was calculated as the g e o m e t r i c m e a n (log~o n + 1). Species diversity was m e a s u r e d from the n u m b e r of species a n d the S h a n n o n diversity index (H') (Pielou, 1977). the equatability b e t w e e n species in the c o m m u n i t y was m e a s u r e d with Pielou's evenness function (J') (Pielou, 1977). All calculations utilized logarithms to base 10. T h e c o m m u n i t y p a r a m e t e r s were calculated for each m o n t h . T h e overall assemblage p a r a m e t e r s for the whole year were calculated from the m e a n catch for the whole sampling p e r i o d (see N a s h , 1988). T h e percentage similarity b e t w e e n successive s a m p l i n g data was calculated using the m e t h o d of W h i t t a k e r and F a i r b a n k s (1958) b a s e d on species a b u n d a n c e data.
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Results Sea-surface temperature T h e o n l y p h y s i c a l v a r i a b l e to b e m e a s u r e d was s e a - s u r f a c e t e m p e r a t u r e . T h e r e was a s e a s o n a l cycle w i t h h i g h t e m p e r a t u r e s (21.9 °C) in J u l y w h i c h d e c l i n e d to a m i n i m u m (14-7 °C) in M a r c h ( F i g u r e 2).
R. S. Santos & R. D. M. Nash
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Month Month Figure 2. Seasonal variations in the community parameters of a shallow water fish assemblage in Porto Pim Bay, Faial, Azores between July 1989 and June 1990. (a) Numbers of species, (b) numbers of individuals caught (solid line) and temperature (dashed line), (c) biomass caught (solid line) and temperature (dashed line), (d) species diversity (/4') (solid line) and evenness (J') (dashed line).
Fish assemblage T w e n t y - f o u r species were caught yielding 39 350 fish or 619.9 kg. T h i s constitutes a yearly average of 223 fish 1 0 0 0 m 2 or 4-5 kg 1 0 0 0 m 2 ( T a b l e 1). Over the four seasons, estimated density was highest in s u m m e r (1.4 individuals m 2) [Figure 3(a)] and lowest in winter. T h e highest b i o m a s s also o c c u r r e d in s u m m e r (27'75 g m 2) followed by a u t u m n , spring and then winter [Figure 3(b)]. T h e d o m i n a n t species with respect to n u m b e r s of individuals a n d b i o m a s s was Chelon labrosus (64 a n d 89% respectively) ( T a b l e 1). T w o species (C. labrosus, Trachinotus ovatus) constituted the majority of the assemblage with respect to n u m b e r s (91%) a n d biomass (97%). O f the species caught, only three species could be c o n s i d e r e d residents of soft-sediment areas and Porto P i m Bay in particular (Dasyatis pastinaca, Echiichthys vipera and Bothus podas). However, there is a certain degree o f d o u b t as to w h e t h e r these species are true residents as a b u n d a n c e s c h a n g e d for D. pastinaca a n d E. vipera suggesting a possible m o v e m e n t out o f the area, a n d the majority o f B. podas were juveniles which p r o b a b l y move off this nursery g r o u n d to the s u r r o u n d i n g rocky areas.
Number of species T h e r e did not a p p e a r to be any clear p a t t e r n in the n u m b e r of species caught at P o r t o P i m [Figure 2(a)]. T h e m a x i m u m n u m b e r o c c u r r e d in S e p t e m b e r (16 species). I n general, there were 9-11 species present each m o n t h .
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TABLE 1. Geometric m e a n abundance and biomass offish caught with a beach seine at Porto Pim Bay over the period July 1989 to June 1990 (no samples in December 1989)
Average number Scientific name
Common name
Dasyatis pastinaca Synodus saurus Sardina pilchardus Belone belone Capros aper Pomatomus saltator Trachurus picturatus Trachitzotus ovatus Pseudocaralzx dentex Mulhts sttrT~ltllettts Pagellus bogaraveo Boops boops Diplodus sargus Chelon labrosus Sarpa salpa Echiichthys vipera Bothus podas Sphoeroides mannoratus Syngnathus acus Conger conger Pagellus acarste Myctophid Unidentified
C o m m o n stingray
Atlantic lizard fish Pilchard Garfish Boarfish Bluefish Blue jack mackerel Pompano Guelly jack Striped red mullet Red sea bream Bogue White sea bream Thicldip grey mullet Salema Lesser weever
Wide-eyed flounder Guinean puffer Greater pipefish Conger eel Auxiliary sea bream Myctophid
Number of species To tal nos/wt
(1000 m - 2 ) 0.29 0.01 6-04 0"07 0'04 0.43 0.74 59.18 0.07 1.15 5.19 0-26 2.29 142-35 0.13 1.06 2.25 0.52 0.01 0.01 0' 16 0.03 0.01 24 222.59
/~verage weight % 0.13 0-01 2.71 0"03 0.02 0"33 0.33 26.59 0.03 0-52 2.33 0.12 1 "03 63.95 0-06 0.47 1.01 0-23 0-01 0.01 0.07 0.01 0.01
(g 1 0 0 0 m -2) 4.81 0-08 43.84 0" 10 0.14 5.63 16.89 349.45 0"35 18.14 10-30 1.75 21.78 3966.65 1.42 4-84 5.81 3-07 + • 0-18 0.31 0.05 0-04
% 0" 11 + 0"98 + + 0.13 0.38 7.84 + 0.41 0.23 0.04 0.49 89-03 0.03 0.11 0" 13 0.07 + + + + +
24 4455.65
+, value <0.01.
Abundance and biomass Numbers of individuals present were at a m a x i m u m in September and again in April [Figure 2(b)]. The peak in abundance occurred just after the summer peak in temperature and the winter minimum. In September, the high catches were caused by substantial increases in the catch of young Sardina pilchardus and Pagellus bogaraveo. In April, the peak was due to increased catches of very young P. bogaraveo. The catches were lowest through the winter (-November-March). The pattern in biomass was different. Catches were highest in August [Figure 2(c)] and declined through September and October to remain fairly low through to May. The very large catch of P. bogaraveo was primarily small juvenile fish, which have very little influence on the biomass.
Species diversity (1t') Species diversity showed relatively little pattern. However, high levels did occur with the peak in numbers of individuals in September [Figure 2(d)]. Apart from a low diversity in April (primarily caused by the dominance of a single species, P. bogaraveo), species diversity remained relatively stable.
584
R. S. Santos & R. D. M. Nash
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Winter Spring Season Figure 3. Seasonal variation in numbers of individuals and biomass per square metre of fish in Porto Pim Bay, Faial, Azores. Seasons are taken as January-March=Winter, April-June = Spring, July-September= Summer and October-December = Autumn.
Evenness (J') In general, evenness was relatively low (<0.4) which reflects the high level of d o m i n a n c e in the assemblage [Figure 2(d)]. T h e equatability o f the assemblage was highest t h r o u g h the winter ( N o v e m b e r - M a r c h ) w h e n there were r e d u c e d n u m b e r s of individuals of d o m i n a n t species. Community parameters and temperature T h e only p a r a m e t e r to show any correlation with t e m p e r a t u r e was total b i o m a s s caught (r=0-81; 9 d.f.; P < 0 ' 0 5 ) . In all other cases ( n u m b e r o f species, n u m b e r o f individuals, H ' and J ' ) , there was no significant correlation ( P > 0 . 0 5 ) . Consistency between sampling dates T h e r e was a reasonably high similarity in the distribution o f n u m b e r s of individuals across species between adjacent s a m p l i n g m o n t h s from June a n d M a r c h with m a j o r changes b e t w e e n M a r c h a n d April ( T a b l e 2). T h e low similarity c o i n c i d e d with the very large influx of very small P. bogaraveo. Similarity was also low b e t w e e n M a y a n d June. This p a t t e r n was not seen in the b i o m a s s d a t a as, in general, similarity b e t w e e n m o n t h s r e m a i n e d high.
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TABLE 2. Percent similarity in the distribution of numbers of individuals and biomass across species between adjacent months for the Porto Pim fish ~assemblage Similarity (%) Months July/August August/September September/October October/November November/January January/February February/March March/April April/May May/June
Number of individuals
Biomass
74.02 59.66 59 "46 87.02 80.61 56.57 74.72 5"29 72.20 25.69
73"51 68-04 45-87 87 "09 83" 17 85-88 54"79 55.73 61-89 59.99
Fish species Four species (C. labrosus, T. ovatus, P. bogaraveo and S. pilchardus) had a major influence on the variation in total numbers of individuals caught through the year. Each of these species had a slightly different pattern in seasonal abundance with a low abundance during the winter months (January-March) being a c o m m o n feature to all species except T. ovatus [Figure 4(a-d)]. T h e greatest abundance of C. labrosus occurred in July with a steady decline to January. T h e highest abundance of T. ovatus occurred in August and both P. bogaraveo and S. pilchardus had elevations in abundance in September. All of these high abundances combined to produce the peak in assemblage abundance in August. In general, all these species abundances were dominated by juveniles. T h e winter rise in abundance in T. ovatus was due to the presence of 0-group fish, also seen in October and November. T h e second peak in abundance in P. bogaraveo was caused by very large catches of 0-group individuals. T h e three most abundant benthic species (D. pastinaca, E. vipera and B. podas) are also covered here, even though their abundances were low by comparison with the dominant, pelagic species. Dasyatis pastinaca and E. vipera appeared to switch in dominance with D. pastinaca abundant in the late spring and s u m m e r (June-August) and E. vipera abundant fxom late s u m m e r through to the following spring ( S e p t e m b e r April) [Figure 5 (a,b)]. Bothus podas (wide-eyed flounder) juveniles arrived in the shallow water in August, causing a peak in abundance and then remained rare for the rest of the sampling period [Figure 5(c)]. T h e abundance of some species, notably S. pilchardus, C. labrosus, B. podas and D. pastinaca, were all positively correlated with the sea-surface temperature in log-log relationships (r=0"69, 0-91, 0"84, 0"70; 9 d.f.; P<0"05 respectively). There was no significant relationship between abundance of T. ovatus nor E. vipera and temperature (r=0"35, 0"07; 9 d.f.; P>0-05 respectively). Discussion T h e fish assemblage in Porto Pim Bay is relatively undisturbed and the bay provides a rare (for the Azores) shallow-water sandy habitat. T h e only major disruptions,
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Month Month Figure 4. Seasonal variation in the abundance (number of individuals) (solid line) and temperature (dashed line) of the dominant fish species in Porto Pim Bay, Faial, Azores between June 1989 and July 1990. (a) Chelon labrosus, (b) Trachinotus ovatus, (c) Pagellus bogaraveo, (d) Sardina pilchardus. outside the normal seasonality of the environment, are large numbers of bathers during daylight through the summer months and occasional fishing operations targeting juvenile pelagic fish. Historically, the bay had high organic inputs from a whaling station at the top of the bay. At present, the assemblage can be considered as exhibiting little stress. The fish assemblage undergoes a strong seasonal change in abundance. This is typical of many shallow-water fish assemblages (McErlean et al., 1973; Quinn, 1980; Lasiak, 1984b; Ross et al., 1987; Nash, 1988; Wright, 1988) with low abundances occurring in the winter. The sampling strategy undertaken here, which covered all tidal states and complete diel cycles, gives us an adequate description of the fish assemblage. The problem of diel variations in fish catches due to either variations in net selectivity, variations in fish behaviour or a combination of both, is well known (e.g. McCleave & Fried, 1975; Horn, 1980; Lasiak, 1984a; Nash et al., 1994a). Very often, catch variability in shallow-water assemblages is also related to a combination of tidal height and light levels (see Nash et al., 1994a). Nash et al. (1994b) argue that, providing the sampling covers the majority of combinations of light levels and tidal height, good descriptions can be obtained for the purposes of determining seasonal patterns in fish assemblage structure. Some of the day/night variability in fish catches in this experiment is reported elsewhere (Nash et al., 1994b). Since there is coverage of most variables (tide,
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Bothus podas. light and seasonality), we believe that (allowing for possible problems with differential selectivity of species by beach seines) this is a reasonable description of the dynamics of this assemblage during 1989-90. One of the striking aspects of this shallow-water area (Porto Pim) is the occurrence of only 24 fish species from 176 samples covering high- and low-water time periods and day and night over 1 year. This can be compared with studies at similar latitudes in shallow-water areas on major continents; e.g. 32 species in Newport Bay, California (Allen, 1982) 37 species in Kuwait Bay, Kuwait (Wright, 1988), 50 species in Kings Bay, South Africa (Lasiak, 1984b) and 46 species in Botany Bay, Australia (Bell et al., 1984). T h e Azores archipelago is relatively isolated from the major continents. As the most northerly of the oceanic archipelagoes in the N o r t h Atlantic, it is in an interesting position. T h e island of Faial is in the Euro-African Basin. T h e large distances from the main continents have major constraints on fish species with benthic eggs and short planktonic lives and as such, the diversity of the fish fauna of the Azores is lower than those of the Madeira and Canary archipelagoes (Patzner et al., 1992). Zoogeographic affinities between the Azores and the Mediterranean have been
588
R. S. Santos & R. D. M. Nash
suggested on a number of occasions (Briggs, 1970, 1974) with m a n y species in the Azores being c o m m o n to the coastal zones of Europe and Africa. There are no known endemic inshore species to the Azores which is not surprising since low evolutionary divergence in shore fish faunas is a common characteristic of North Atlantic islands (Briggs, 1966, 1970). The Porto Pim Bay assemblage only has three abundant benthic species (D. pastinaca, E. vipera and B. podas). T h e presence of only one flatfish species is surprising when compared with adjacent areas, e.g. west coast of Portugal. Bothus podas in the Azores is not confined to the rocky substrata whereas, on the continent, it is generally found almost solely on rocky substrata (Nash et al., 1991). T h e interactions between the three main benthic species has not been investigated as yet. Another surprising factor was the lack of gobies in the assemblage. Pomatoschistus pictus occurs in the area (Patzner et al., 1992) but, unlike more northern areas, (e.g. Nash, 1988) does not appear to frequent the open sand areas and occur in beach seine samples. The only comparison between the sandy and rocky intertidal regions in the Azores is given by Santos et al. (1994). In general, the rocky intertidal pools are dominated by resident blenny species (e.g. Parablennius sanguinolentus parvicornis, Coryphoblennius galerita and Lipophrys trigloides). There are, however, a n u m b e r of transient species which occur in these rocky shore pools, which also occur in the sandy bay, e.g.C, labrosus and Diplodus sargus. O f the remaining species caught in the beach seine all species, with the exception of E. vipera and D. pastinaca have been observed as both adults and/or juveniles frequenting subtidal rocky habitats. This clearly shows the general paucity of the obligate shallow-water, soft-sediment fish fauna in the area. The seasonal change is estimated biomass per square metre in Porto Pim Bay had a similar pattern to that recorded for a Gulf of Mexico surf zone fish assemblage (Ross et al., 1987). The estimated biomass per square metre (27"8 g m - 2 in the summer months) was in the upper part of the range reported for a n u m b e r of different marine near shore environments (0"8-57-0 g m - 2, given in Ross et al., 1987), but considerably lower than seen in some selected habitats, i.e. rocky reef, coral reef and estuarine areas (Whitfield, 1993). However, the estimated m a x i m u m density (1.4 individuals m - 2) was in the lower part of the range reported by Ross et al. (1987) for a number of other areas (0-18-9.0 individuals m 2). There are many problems associated with comparisons of fish standing stock between areas due to the range of sampling techniques used, species compositions and hence assemblage behaviour (Whitfield, 1993). However, this comparison suggests that the standing stock in Porto Pim is in the upper range reported for other beach seine studies. The study in Porto Pim Bay was only over 1 year and the inter-annual variation in abundance is unknown. This fish assemblage, as seen from beach seine samples, is dominated by pelagic species with respect to both numbers of individuals and biomass. T h e consequences are a relatively low level of equatability (evenness) throughout the year. This is quite common in these shallow-water areas from temperate to subtropic zones (Livingston, 1976; Lasiak, 1984b; Nash, 1988). T h e species diversity (H') at Porto Pim was remarkably high when compared to Morro Bay, California for instance (Horn, 1980). The lowest values, which occurred in April, were similar to the highest values reached in this southern Californian fish assemblage. There was a slight seasonality in species diversity with the highest levels occurring in the summer (June-October). Similar variations have been seen in temperate shallow-water assemblages (Nash, 1988) and
Azorean shallow-water fish
589
sublittoral fish assemblages (Nash & Gibson, 1982). In general, the trends are not particularly clear in any of these studies. In efforts to characterize fish assemblages and assess the influence of stress and/or pollution, combinations of species diversity parameters and seasonal similarity in species compositions have been used (see Horn, 1980). T h e suggestion is that unstressed fish assemblages should show a high species diversity with low similarities in species composition between seasons. T h e combination of a very high diversity and the low levels of similarity between some months adds weight to the suggestion that this is an unstressed fish assemblage and that there is little human-induced pollution stress. T h e major changes in the species compositions in this assemblage occurred during the autumn and spring. In both cases there was an influx of juvenile P. bogaraveo. Tiffs species spawns between January and April in this region (Krug, 1990). T h e very large influx in April was probably due to the new O-group fish coming in to the nursery area. T h e smaller elevation in abundance in September represents the previous year's juveniles returning to the shallow embayments, possibly in an effort to avoid large pelagic predators. T h e peak in September was also caused by juvenile S. pilchardus in the bay. Juveniles are known to shoal in shallow water (Wheeler, 1969). Spawning generally occurs over the autumn and winter period (Whitehead et al., 1986). Therefore the influx of juveniles is possibly a method of avoiding the larger offshore pelagic predators (Wright, 1988). T h e other two dominant pelagic species in the bay (C. labrosus and T. ovatus) both have their highest abundance in July and August. These species are spring spawners (Wheeler, 1969) with the major recruitment of the new year class to the embayments occurring in the autumn. In the case of T. ovatus, there was a secondary arrival of young during the winter. Unfortunately there has been very little work done on these species in the Azores region, therefore m u c h of the general biology is unknown. T h e major benthic species were at a m u c h lower level of abundance than the dominant pelagics. T h e two carnivorous species appear to dominate the bay at different times. Dasyatis pastinaca was prevalent during the s u m m e r months, being replace by E. vipera for the rest of the year. Presumably there are very few fish species which would predate on either of these species. In the shallow coastal waters of south Africa, D. pastinaca is classed as a spring/summer resident (Lasiak, 1984b). T h e flatfish, B. podas, generally recruited to the bay in September as newly metamorphosing fish (see Nash et al., 1991). T h e numbers of juveniles declined rapidly but there were adults in the bay for most of the year. However, this species also occurs on hard substrata (Nash et al., 1991) and changes in abundance may be caused by movements between these two habitats. T o a certain extent, thee three species along with S. saurus and Sphoeroides marmoratus can be considered as residents while most of the others are migrant species. T h e y use the bay as either a nursery ground, as a rich feeding ground by predators (e.g. adult Pomatomus saltator, this species also occurs as juveniles in the bay), or as an occasional habitat for fishes from deep offshore waters (e.g. myctophids). We consider S. saurus may be a resident because it buries itself in the sand. Its low abundance in the seine samples is primarily due to its ability to bury. Individuals have been seen throughout the bay during diving studies (our observations). Sphoeroides marmoratus is also considered a resident because it appears to have a certain degree of fidelity to location and probably does not undertake large migrations out of the bay for spawning. In conclusion, there is a distinct seasonality to the community structure of the fish assemblage in Porto Pim Bay. T h e highest biomass occurs in the s u m m e r and this is
590
R. S. Santos & R. D. M. Nash
comparable to results obtained from other inshore marine environments. The fish f a u n a l diversity, w h e n c o m p a r e d w i t h a r e a s o n c o n t i n e n t s a n d t h e p r e s e n c e j u v e n i l e fish i n t h i s e n v i r o n m e n t , w h i c h is r a r e i n t h e A z o r e s , w a r r a n t s i n v e s t i g a t i o n s to u n d e r s t a n d t h e d y n a m i c s o f fish s p e c i e s a n d a s s e m b l a g e s i n
reduced of many further shallow
water in the middle of the Atlantic Ocean.
Acknowledgements T h e a u t h o r s are g r a t e f u l t o t h e B r i t i s h C o u n c i l ( P o r t u g a l ) , I N I C ( P o r t u g a l ) a n d JNICT/Ci~ncia (Portugal), the Azorean Regional Department of Fisheries (SRAP/DRP) and Azorean Department of the Environment (SRTA/DRA) for funding associated with this work. This project was initiated through the Department of Oceanography and Fisheries (University of the Azores)/Port Erin Marine Laboratory (University of L i v e r p o o l ) E x p e d i t i o n to t h e A z o r e s i n J u n e / J u l y 1 9 8 9 a n d o u r g r a t i t u d e is e x t e n d e d t o all s p o n s o r s o f t h a t e x p e d i t i o n . T h e a u t h o r s a c k n o w l e d g e t h e field a n d l a b o r a t o r y support given by Olavo Amaral, Vitor Rosa, Noberto Serpa, Fatima Serpa and Debbie Neves. Contribution No. 24 from the 'Expedition Azores 1989'.
References Ali, T. S. & Hussain, N. A. 1990 Composition and seasonal fluctuations of intertidal fish assemblage in Kohr al-Zubair, Northwestern Arabian Gulf. Journal of Applied Ichthyology 6, 24-36. Allen, L. G. 1982 Seasonal abundance, composition, and productivity of the littoral fish assemblage in upper Newport Bay, California. Fishery Bulletin 80, 769-790. Allen, L. G. & DeMartini, E. E. 1983 Temporal and spatial patterns of nearshore distribution and abundance of the pelagic fishes of San Onofre-Oceanside, California. Fishery Bulletin gl, 569-586. Allen, L. G., Horn, M. H., Edmonds, F. A. & Usui, C. A. 1983 Structure and seasonal dynamics of the fish assemblages in the Cabrillo beach area of Los Angeles harbor, California. Bulletin of the South California Acadamy of Science 82, 47-70. Bell, J. D., Pollard, D. A., Burchmore, J. J., Pease, B. C. & Middleton, M. J. 1984 Structure of a fish community in a temperate tidal mangrove creek in Botany Bay, New South Wales. Australian Journal of Marine and Freshwater Research 35, 33-46. Briggs, J. C. 1966 Oceanic islands, endemism and marine paleotemperatures. Systematic Zoology 15, 153-163. Briggs, J. C. 1970 A faunal history of the North Atlantic Ocean. Systematic Zoology 19, 19-34. Briggs, J. C. 1974 Marine Zoogeography. MacGraw-Hill, New York. Horn, M. H. 1980 Diel and seasonal variation in abundance and diversity of shallow-water fish populations in Morro Bay, California. Fishery Bullethz 78, 759-770. Krug, H. M. S. 1990 The Azorean blackspot seabream, Pagellus bogaraveo (Brtinnich, 1768) (Teleostei: Sparidae): Reproductive cycle, hermaphroditism, maturity and fecundity. Cybium 14, 151-159. Kubecka, J. & Bohm, M. 1991 The fish fauna of the Jordan reservoir, one of the oldest manmade lakes in Central Europe. Journal ofFish Biology 38, 935-950. Lasiak, T. A. 1984a Structural aspects of the surf-zone fish assemblage at King's beach, Algoa Bay, South Africa: Short-term fluctuations. Estuarine, Coastal and Shelf Science 18, 347-360. Lasiak, T. A. 1984b Structural aspects of the surf-zone fish assemblage at King's beach, Algoa Bay, South Africa: Long-term fluctuations. Estuarine, Coastal and Shelf Science 18, 459-483. Livingston, R. J. 1976 Diurnal and seasonal fluctuations of organisms in a north Florida estuary. Estuarine and Coastal Marine Science 4, 373-400. McCleave, J. C. & Fried, S. M. 1975. Nighttime catches of fishes in a tidal cove in Montsweag Bay near Wiscasset, Maine. Transactions of the American Fisheries Society 104, 153-163. McErlean, A. J., O'Connor, S. G., Milhursky, J. A. & Gibson, C. I. 1973 Abundance, diversity and seasonal patterns of estuarine fish populations. Estuarine and Coastal Marine Science 1, 19-36. Nash, R. D. M. 1988 The effects of disturbance and severe seasonal fluctuations in environmental conditions on north temperate shallow-water fish assemblages. Estuarine, Coastal and Shelf Science 26, 123-135. Nash, R. D. M. & Gibson, R. N. 1982 Seasonal fluctuations and compositions of two populations of small demersal fishes on the west coast of Scotland. Estuarine, Coastal and Shelf Science 15, 485-495.
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Nash, R. D. M. & Santos, R. S. 1993 The occurrence of the lesser weever in the Azores. Journal offish Biology 43, 317-319. Nash, R. D. M., Hawkins, S. J., Fisher, E. & Hill, A. 1990 Diel fluctuations and 15opulation structure of a sandy beach shallow-water fish assemblage. In Expediqao Aqores:89/Relatorio Preliminar (DOP/UA), Vol. II, (Santos, R. S., ed.). Universidade dos Aqores, pp. 233-250. Nash, R. D. M., Geffen, A. J. & Santos, R. S. 1991 The wide-eyed flounder, Bothus podas DeLaroche, a singular flatfish in varied shallow-water habitats of the Azores. Netherlands Journal of Sea Research 27, 367-373. Nash, R. D. M., Santos, R. S., Geffen, A. J., Hughes, G. & Ellis, T. R. 1994a Diel variability in catch rate of juvenile flatfish on two small nursery grounds (Port Erin Bay, Isle of Man and Porto Pim Bay, Faial, Azores). Journal ofFish Biology 44, 35-45. Nash, R. D. M., Santos, R. S. & Hawkins, S. J. 1994b Diel fluctuations of a sandy beach fish assemblage at Porto Pim, Azores. Arquip~lago-Life and Marine Science 12A, 75-86. Patzner, R. A., Santos, R. S., Rr, P. & Nash, R. D. M. 1992 Littoral fishes of the Azores: An annotated checklist of fishes observed during the "Expedition Azores 1989". Arquip~lago-Life and Earth Sciences 10, 101-111. Pielou, E. C. 1977 Mathematical Ecology. Wiley, New York. Quinn, N. J. 1980 Analysis of temporal changes in fish assemblages in Serpentine Creek, Queensland. Environmental Biology of Fishes 5, 117 133. Ross, S. T., McMichael, R. H. & Ruple, D. L. 1987 Seasonal and diel variation in the standing crop of fishes and macroinvertebrates from a Gulf of Mexico surf zone. Estuarine, Coastal and Shelf Science 25, 391-412. Santos, R. S., Nash, R. D. M. & Hawkins, S. J. 1994 Fish assemblages on intertidal shores of the island of Faial, Azores. Arquipklago-Life and Marine Sciences 12A, 87-100. Wheeler, A. 1969 The Fishes of the British Isles and North-west Europe. Macmillan, London. Whitehead, P. J. P., Bauchot, M.-L., Hureau, J.-C., Nielsen, J. & Tortonese, E. (eds.) 1.986 Fishes of the North-eastern Atlantic and Mediterranean, Vol. I-III. UNESCO, Chaucer Press, U.K. Whitfield, A. K. 1993 Fish biomass estimates from the littoral zone of an estuarine coastal lake. Estuaries 16, 280-289. Whittaker, R. H. & Fairbanks, C. W. 1958 A study of plankton copepod communities in the Columbian Basin, south-eastern Washington. Ecology 39, 46-65. Wright, J. M. 1988 Seasonal and spatial differences in the fish assemblage of the non-estuarine Sulaibikhat Bay, Kuwait. Marine Biology 100, 13-20. Wright, J. M. 1989 Diel variation and seasonal consistency in the fish assemblage of the non-estuarine Sulaibikhat Bay, Kuwait. Marine Biology 102, 135-142.
Seasonal Changes in a Sandy B e a c h Fish A s s e m b l a g e at Porto P i m , Faial,-Azores
Ricardo S. Santos a and Richard D. M. Nash b ~Departamento de Oceanografia e Pescas, Universidade dos A~ores, PT-9900 Horta, Faial, Azores, Portugal and bPort Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man IM9 6JA, U.K. Received 19 August 1993 and in revised form 13 September 1994
Keywords: Azores; shallow water; fish; seasonal A shallow-water fish assemblage, over a soft, sandy substratum at Porto Pim Bay on the island of Faial, Azores, was sampled at monthly intervals with a beach seine. Sampling was undertaken at 3-h intervals over 24 h'on a monthly basis between July 1989 and June 1990. There was seasonal variation in the density of individuals and biomass with the highest values in summer (1 "4 m - 2 and 27-75 g m 2 respectively). A total of 24 species were observed and the assemblage was dominated by four species (Chelon labrosus, Sardina pilchardus, Pagellus bogaraveo, Trachinotus ovatus). Species diversity (H') was high throughout the year but equatability (_7') was low, due to the dominance of a few species. The largest change in the assemblage structure occurred between March and April. Only a few species could be classified as resident, with the two predators (Dasyatis pastinaca and Echiichthys vipera) being abundant at different times of the year. Most of the species observed were juvenile fish, many with an offshore distribution as adults, using Porto Pim Bay as a nursery ground. c~ 1995 Academic Press Limited
Introduction T h e r e have b e e n a n u m b e r of studies which have e x a m i n e d the seasonal variations in shallow-water fish assemblage structure o n soft sediments ( H o r n , 1980; Allen & D e M a r t i n i , 1983; Allen et al., 1983; Lasiak, 1984b; N a s h , 1988; Wright, 1988, 1989; Ali & H u s s a i n , 1990). T h e s e studies vary from descriptions of the fish assemblages a n d their d y n a m i c s (e.g. H o r n , 1980) to a n e x a m i n a t i o n of the effects of severe seasonal changes in e n v i r o n m e n t a l c o n d i t i o n s (Mash, 1988). Shallow-water, s a n d - b o t t o m areas in the Azores are relatively scarce a n d it is only recently that this habitat has b e e n studied (Nash et al., 1990, 1991, 1994a; N a s h & Santos, 1993). T h e r e is a fairly large range in the potential density of fish a n d s t a n d i n g crops in shallow-water areas (Ross et al., 1987), a n d it is u n k n o w n whether the relatively rare shallow-water habitat in the Azores is high or low with respect to these other temperate a n d subtropical areas. T h e Azores islands are also generally well separated a n d as a group they are approximately 1700 k m from the nearest c o n t i n e n t . D u e to the close proximity of very deep water, there is also the possibility of interactions b e t w e e n coastal a n d oceanic species as well as shallow-water a n d deep-water species. 0272-7714/95/050579+ 13 $12.00/0
© 1995 Academic Press Limited
580
R. S. Santos & R. D. M. Nash
This study provides a description of the seasonal fluctuations in a shallow-water, soft-sediment fish assemblage, and c o m p a r e s this with o t h e r studies u n d e r t a k e n elsewhere b o t h in other subtropical and in t e m p e r a t e areas.
S t u d y site T h e Azores are a small group of islands (which cover a fairly large area) located along the m i d - A t l a n t i c ridge in the region of 38°N 28°W. T h e s a m p l i n g site, Porto Pim, is located on the south-east corner of the island of Faial (Figure 1). T h e beach is small (approximately 280 m in length) facing south-west. T h e n o r t h a n d south coasts of the beach area are rocky. T h e southern b o r d e r is the edge of the M o n t e da G u i a p r o t e c t e d area. T h e m a x i m u m tidal range here is 1 m. T h e m a x i m u m horizontal intertidal area of the beach at low water is a p p r o x i m a t e l y 30 m.
Methods Sampling was u n d e r t a k e n with a 20 m b e a c h seine (32 m m stretch m e s h in the wings reducing t h r o u g h 15 m m to 8 m m in the centre). T w o sets of the b e a c h seine were m a d e at 3-h intervals for 24 h on each s a m p l i n g date. S a m p l i n g c o m m e n c e d in July 1989 a n d finished in June 1990 at a p p r o x i m a t e l y 1 - m o n t h intervals. E x t r e m e l y b a d w e a t h e r in D e c e m b e r 1989 p r e v e n t e d s a m p l i n g on that occasion. T h e b e a c h seine was fitted with 25 m hauling ropes and set parallel to the beach. T h e net swept an area from a p p r o x i m a t e l y 4 m d e p t h to the shore, with each haul having an effective fishing area o f 291 m 2 [(using the m e t h o d o l o g y of K u b e c k a & B o h m (1991)]. N o estimates o f overall efficiency are available for this net on this beach. W a t e r t e m p e r a t u r e was o b t a i n e d from H o r t a H a r b o u r as part o f the general daily m o n i t o r i n g p r o g r a m m e c o n d u c t e d by the D e p a r t m e n t of O c e a n o g r a p h y a n d Fisheries, University of the Azores, Horta. T h e data given here are for the specific s a m p l i n g dates used. All species were identified using W h i t e h e a d et al. (1986). T h e total n u m b e r of individuals and total weight for each species in each haul was obtained. T h e lengths (to the nearest 1 m m ) a n d individual weights (to the nearest 1 g) (using a s u b s a m p l e of up to 100 individuals where necessary) were m e a s u r e d for each sample. In this report, all m o n t h l y catches are r e p o r t e d as a s u m m a t i o n of the two hauls taken at each s a m p l i n g time and the eight samples taken over the 24-h p e r i o d (i.e. 16 samples). Average catch (numbers and biomass) was calculated as the g e o m e t r i c m e a n (log~o n + 1). Species diversity was m e a s u r e d from the n u m b e r of species a n d the S h a n n o n diversity index (H') (Pielou, 1977). the equatability b e t w e e n species in the c o m m u n i t y was m e a s u r e d with Pielou's evenness function (J') (Pielou, 1977). All calculations utilized logarithms to base 10. T h e c o m m u n i t y p a r a m e t e r s were calculated for each m o n t h . T h e overall assemblage p a r a m e t e r s for the whole year were calculated from the m e a n catch for the whole sampling p e r i o d (see N a s h , 1988). T h e percentage similarity b e t w e e n successive s a m p l i n g data was calculated using the m e t h o d of W h i t t a k e r and F a i r b a n k s (1958) b a s e d on species a b u n d a n c e data.
Azorean shallow-waterfish
581
29°W
28°W
(
27°W
Graciosa
39°N S. Jorge Faial Terceira 38° 30 ~N
/ 20 ~,~
lO
/ HORTA
Pim
w-10
\20 I
500 m
Monte da Guia
I
Figure 1. Map showing the central islands of the Azores and the sampling location of Porto Pim Bay on the island of Faial. Numbers indicate depth in metres.
Results Sea-surface temperature T h e o n l y p h y s i c a l v a r i a b l e to b e m e a s u r e d was s e a - s u r f a c e t e m p e r a t u r e . T h e r e was a s e a s o n a l cycle w i t h h i g h t e m p e r a t u r e s (21.9 °C) in J u l y w h i c h d e c l i n e d to a m i n i m u m (14-7 °C) in M a r c h ( F i g u r e 2).
R. S. Santos & R. D. M. Nash
582
20 (a) 18
200
16
160'
/m
~
14/m
~
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. (c)
.o.
mX
~
\
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,22
20
-..
80
',
/ "
Z
4
2 0
40
I I I (b) •
o,ooo "l/
"~
/4.
"~"
~ 6000 \,.n/
t I I I I
I
•
I t I I
0
•
l
5~
A t //
4
1 *.0 ,
(d)
'r-,," \ 0.-
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•
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"d
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~'/ ~'/ "~ ,~
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• 0"1 ~ 0 1 I I I I I I I I I 0.0 1
Month Month Figure 2. Seasonal variations in the community parameters of a shallow water fish assemblage in Porto Pim Bay, Faial, Azores between July 1989 and June 1990. (a) Numbers of species, (b) numbers of individuals caught (solid line) and temperature (dashed line), (c) biomass caught (solid line) and temperature (dashed line), (d) species diversity (/4') (solid line) and evenness (J') (dashed line).
Fish assemblage T w e n t y - f o u r species were caught yielding 39 350 fish or 619.9 kg. T h i s constitutes a yearly average of 223 fish 1 0 0 0 m 2 or 4-5 kg 1 0 0 0 m 2 ( T a b l e 1). Over the four seasons, estimated density was highest in s u m m e r (1.4 individuals m 2) [Figure 3(a)] and lowest in winter. T h e highest b i o m a s s also o c c u r r e d in s u m m e r (27'75 g m 2) followed by a u t u m n , spring and then winter [Figure 3(b)]. T h e d o m i n a n t species with respect to n u m b e r s of individuals a n d b i o m a s s was Chelon labrosus (64 a n d 89% respectively) ( T a b l e 1). T w o species (C. labrosus, Trachinotus ovatus) constituted the majority of the assemblage with respect to n u m b e r s (91%) a n d biomass (97%). O f the species caught, only three species could be c o n s i d e r e d residents of soft-sediment areas and Porto P i m Bay in particular (Dasyatis pastinaca, Echiichthys vipera and Bothus podas). However, there is a certain degree o f d o u b t as to w h e t h e r these species are true residents as a b u n d a n c e s c h a n g e d for D. pastinaca a n d E. vipera suggesting a possible m o v e m e n t out o f the area, a n d the majority o f B. podas were juveniles which p r o b a b l y move off this nursery g r o u n d to the s u r r o u n d i n g rocky areas.
Number of species T h e r e did not a p p e a r to be any clear p a t t e r n in the n u m b e r of species caught at P o r t o P i m [Figure 2(a)]. T h e m a x i m u m n u m b e r o c c u r r e d in S e p t e m b e r (16 species). I n general, there were 9-11 species present each m o n t h .
Azorean shallow-water fish
583
TABLE 1. Geometric m e a n abundance and biomass offish caught with a beach seine at Porto Pim Bay over the period July 1989 to June 1990 (no samples in December 1989)
Average number Scientific name
Common name
Dasyatis pastinaca Synodus saurus Sardina pilchardus Belone belone Capros aper Pomatomus saltator Trachurus picturatus Trachitzotus ovatus Pseudocaralzx dentex Mulhts sttrT~ltllettts Pagellus bogaraveo Boops boops Diplodus sargus Chelon labrosus Sarpa salpa Echiichthys vipera Bothus podas Sphoeroides mannoratus Syngnathus acus Conger conger Pagellus acarste Myctophid Unidentified
C o m m o n stingray
Atlantic lizard fish Pilchard Garfish Boarfish Bluefish Blue jack mackerel Pompano Guelly jack Striped red mullet Red sea bream Bogue White sea bream Thicldip grey mullet Salema Lesser weever
Wide-eyed flounder Guinean puffer Greater pipefish Conger eel Auxiliary sea bream Myctophid
Number of species To tal nos/wt
(1000 m - 2 ) 0.29 0.01 6-04 0"07 0'04 0.43 0.74 59.18 0.07 1.15 5.19 0-26 2.29 142-35 0.13 1.06 2.25 0.52 0.01 0.01 0' 16 0.03 0.01 24 222.59
/~verage weight % 0.13 0-01 2.71 0"03 0.02 0"33 0.33 26.59 0.03 0-52 2.33 0.12 1 "03 63.95 0-06 0.47 1.01 0-23 0-01 0.01 0.07 0.01 0.01
(g 1 0 0 0 m -2) 4.81 0-08 43.84 0" 10 0.14 5.63 16.89 349.45 0"35 18.14 10-30 1.75 21.78 3966.65 1.42 4-84 5.81 3-07 + • 0-18 0.31 0.05 0-04
% 0" 11 + 0"98 + + 0.13 0.38 7.84 + 0.41 0.23 0.04 0.49 89-03 0.03 0.11 0" 13 0.07 + + + + +
24 4455.65
+, value <0.01.
Abundance and biomass Numbers of individuals present were at a m a x i m u m in September and again in April [Figure 2(b)]. The peak in abundance occurred just after the summer peak in temperature and the winter minimum. In September, the high catches were caused by substantial increases in the catch of young Sardina pilchardus and Pagellus bogaraveo. In April, the peak was due to increased catches of very young P. bogaraveo. The catches were lowest through the winter (-November-March). The pattern in biomass was different. Catches were highest in August [Figure 2(c)] and declined through September and October to remain fairly low through to May. The very large catch of P. bogaraveo was primarily small juvenile fish, which have very little influence on the biomass.
Species diversity (1t') Species diversity showed relatively little pattern. However, high levels did occur with the peak in numbers of individuals in September [Figure 2(d)]. Apart from a low diversity in April (primarily caused by the dominance of a single species, P. bogaraveo), species diversity remained relatively stable.
584
R. S. Santos & R. D. M. Nash
2°l 1.5 z
1.0
0.5
Summer
Autumn
Summer
Autumn
Winter Season
Spring
40
E
h~
20
I 10
0
Winter Spring Season Figure 3. Seasonal variation in numbers of individuals and biomass per square metre of fish in Porto Pim Bay, Faial, Azores. Seasons are taken as January-March=Winter, April-June = Spring, July-September= Summer and October-December = Autumn.
Evenness (J') In general, evenness was relatively low (<0.4) which reflects the high level of d o m i n a n c e in the assemblage [Figure 2(d)]. T h e equatability o f the assemblage was highest t h r o u g h the winter ( N o v e m b e r - M a r c h ) w h e n there were r e d u c e d n u m b e r s of individuals of d o m i n a n t species. Community parameters and temperature T h e only p a r a m e t e r to show any correlation with t e m p e r a t u r e was total b i o m a s s caught (r=0-81; 9 d.f.; P < 0 ' 0 5 ) . In all other cases ( n u m b e r o f species, n u m b e r o f individuals, H ' and J ' ) , there was no significant correlation ( P > 0 . 0 5 ) . Consistency between sampling dates T h e r e was a reasonably high similarity in the distribution o f n u m b e r s of individuals across species between adjacent s a m p l i n g m o n t h s from June a n d M a r c h with m a j o r changes b e t w e e n M a r c h a n d April ( T a b l e 2). T h e low similarity c o i n c i d e d with the very large influx of very small P. bogaraveo. Similarity was also low b e t w e e n M a y a n d June. This p a t t e r n was not seen in the b i o m a s s d a t a as, in general, similarity b e t w e e n m o n t h s r e m a i n e d high.
Azorean shallow-waterfish
585
TABLE 2. Percent similarity in the distribution of numbers of individuals and biomass across species between adjacent months for the Porto Pim fish ~assemblage Similarity (%) Months July/August August/September September/October October/November November/January January/February February/March March/April April/May May/June
Number of individuals
Biomass
74.02 59.66 59 "46 87.02 80.61 56.57 74.72 5"29 72.20 25.69
73"51 68-04 45-87 87 "09 83" 17 85-88 54"79 55.73 61-89 59.99
Fish species Four species (C. labrosus, T. ovatus, P. bogaraveo and S. pilchardus) had a major influence on the variation in total numbers of individuals caught through the year. Each of these species had a slightly different pattern in seasonal abundance with a low abundance during the winter months (January-March) being a c o m m o n feature to all species except T. ovatus [Figure 4(a-d)]. T h e greatest abundance of C. labrosus occurred in July with a steady decline to January. T h e highest abundance of T. ovatus occurred in August and both P. bogaraveo and S. pilchardus had elevations in abundance in September. All of these high abundances combined to produce the peak in assemblage abundance in August. In general, all these species abundances were dominated by juveniles. T h e winter rise in abundance in T. ovatus was due to the presence of 0-group fish, also seen in October and November. T h e second peak in abundance in P. bogaraveo was caused by very large catches of 0-group individuals. T h e three most abundant benthic species (D. pastinaca, E. vipera and B. podas) are also covered here, even though their abundances were low by comparison with the dominant, pelagic species. Dasyatis pastinaca and E. vipera appeared to switch in dominance with D. pastinaca abundant in the late spring and s u m m e r (June-August) and E. vipera abundant fxom late s u m m e r through to the following spring ( S e p t e m b e r April) [Figure 5 (a,b)]. Bothus podas (wide-eyed flounder) juveniles arrived in the shallow water in August, causing a peak in abundance and then remained rare for the rest of the sampling period [Figure 5(c)]. T h e abundance of some species, notably S. pilchardus, C. labrosus, B. podas and D. pastinaca, were all positively correlated with the sea-surface temperature in log-log relationships (r=0"69, 0-91, 0"84, 0"70; 9 d.f.; P<0"05 respectively). There was no significant relationship between abundance of T. ovatus nor E. vipera and temperature (r=0"35, 0"07; 9 d.f.; P>0-05 respectively). Discussion T h e fish assemblage in Porto Pim Bay is relatively undisturbed and the bay provides a rare (for the Azores) shallow-water sandy habitat. T h e only major disruptions,
586
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Month Month Figure 4. Seasonal variation in the abundance (number of individuals) (solid line) and temperature (dashed line) of the dominant fish species in Porto Pim Bay, Faial, Azores between June 1989 and July 1990. (a) Chelon labrosus, (b) Trachinotus ovatus, (c) Pagellus bogaraveo, (d) Sardina pilchardus. outside the normal seasonality of the environment, are large numbers of bathers during daylight through the summer months and occasional fishing operations targeting juvenile pelagic fish. Historically, the bay had high organic inputs from a whaling station at the top of the bay. At present, the assemblage can be considered as exhibiting little stress. The fish assemblage undergoes a strong seasonal change in abundance. This is typical of many shallow-water fish assemblages (McErlean et al., 1973; Quinn, 1980; Lasiak, 1984b; Ross et al., 1987; Nash, 1988; Wright, 1988) with low abundances occurring in the winter. The sampling strategy undertaken here, which covered all tidal states and complete diel cycles, gives us an adequate description of the fish assemblage. The problem of diel variations in fish catches due to either variations in net selectivity, variations in fish behaviour or a combination of both, is well known (e.g. McCleave & Fried, 1975; Horn, 1980; Lasiak, 1984a; Nash et al., 1994a). Very often, catch variability in shallow-water assemblages is also related to a combination of tidal height and light levels (see Nash et al., 1994a). Nash et al. (1994b) argue that, providing the sampling covers the majority of combinations of light levels and tidal height, good descriptions can be obtained for the purposes of determining seasonal patterns in fish assemblage structure. Some of the day/night variability in fish catches in this experiment is reported elsewhere (Nash et al., 1994b). Since there is coverage of most variables (tide,
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Bothus podas. light and seasonality), we believe that (allowing for possible problems with differential selectivity of species by beach seines) this is a reasonable description of the dynamics of this assemblage during 1989-90. One of the striking aspects of this shallow-water area (Porto Pim) is the occurrence of only 24 fish species from 176 samples covering high- and low-water time periods and day and night over 1 year. This can be compared with studies at similar latitudes in shallow-water areas on major continents; e.g. 32 species in Newport Bay, California (Allen, 1982) 37 species in Kuwait Bay, Kuwait (Wright, 1988), 50 species in Kings Bay, South Africa (Lasiak, 1984b) and 46 species in Botany Bay, Australia (Bell et al., 1984). T h e Azores archipelago is relatively isolated from the major continents. As the most northerly of the oceanic archipelagoes in the N o r t h Atlantic, it is in an interesting position. T h e island of Faial is in the Euro-African Basin. T h e large distances from the main continents have major constraints on fish species with benthic eggs and short planktonic lives and as such, the diversity of the fish fauna of the Azores is lower than those of the Madeira and Canary archipelagoes (Patzner et al., 1992). Zoogeographic affinities between the Azores and the Mediterranean have been
588
R. S. Santos & R. D. M. Nash
suggested on a number of occasions (Briggs, 1970, 1974) with m a n y species in the Azores being c o m m o n to the coastal zones of Europe and Africa. There are no known endemic inshore species to the Azores which is not surprising since low evolutionary divergence in shore fish faunas is a common characteristic of North Atlantic islands (Briggs, 1966, 1970). The Porto Pim Bay assemblage only has three abundant benthic species (D. pastinaca, E. vipera and B. podas). T h e presence of only one flatfish species is surprising when compared with adjacent areas, e.g. west coast of Portugal. Bothus podas in the Azores is not confined to the rocky substrata whereas, on the continent, it is generally found almost solely on rocky substrata (Nash et al., 1991). T h e interactions between the three main benthic species has not been investigated as yet. Another surprising factor was the lack of gobies in the assemblage. Pomatoschistus pictus occurs in the area (Patzner et al., 1992) but, unlike more northern areas, (e.g. Nash, 1988) does not appear to frequent the open sand areas and occur in beach seine samples. The only comparison between the sandy and rocky intertidal regions in the Azores is given by Santos et al. (1994). In general, the rocky intertidal pools are dominated by resident blenny species (e.g. Parablennius sanguinolentus parvicornis, Coryphoblennius galerita and Lipophrys trigloides). There are, however, a n u m b e r of transient species which occur in these rocky shore pools, which also occur in the sandy bay, e.g.C, labrosus and Diplodus sargus. O f the remaining species caught in the beach seine all species, with the exception of E. vipera and D. pastinaca have been observed as both adults and/or juveniles frequenting subtidal rocky habitats. This clearly shows the general paucity of the obligate shallow-water, soft-sediment fish fauna in the area. The seasonal change is estimated biomass per square metre in Porto Pim Bay had a similar pattern to that recorded for a Gulf of Mexico surf zone fish assemblage (Ross et al., 1987). The estimated biomass per square metre (27"8 g m - 2 in the summer months) was in the upper part of the range reported for a n u m b e r of different marine near shore environments (0"8-57-0 g m - 2, given in Ross et al., 1987), but considerably lower than seen in some selected habitats, i.e. rocky reef, coral reef and estuarine areas (Whitfield, 1993). However, the estimated m a x i m u m density (1.4 individuals m - 2) was in the lower part of the range reported by Ross et al. (1987) for a number of other areas (0-18-9.0 individuals m 2). There are many problems associated with comparisons of fish standing stock between areas due to the range of sampling techniques used, species compositions and hence assemblage behaviour (Whitfield, 1993). However, this comparison suggests that the standing stock in Porto Pim is in the upper range reported for other beach seine studies. The study in Porto Pim Bay was only over 1 year and the inter-annual variation in abundance is unknown. This fish assemblage, as seen from beach seine samples, is dominated by pelagic species with respect to both numbers of individuals and biomass. T h e consequences are a relatively low level of equatability (evenness) throughout the year. This is quite common in these shallow-water areas from temperate to subtropic zones (Livingston, 1976; Lasiak, 1984b; Nash, 1988). T h e species diversity (H') at Porto Pim was remarkably high when compared to Morro Bay, California for instance (Horn, 1980). The lowest values, which occurred in April, were similar to the highest values reached in this southern Californian fish assemblage. There was a slight seasonality in species diversity with the highest levels occurring in the summer (June-October). Similar variations have been seen in temperate shallow-water assemblages (Nash, 1988) and
Azorean shallow-water fish
589
sublittoral fish assemblages (Nash & Gibson, 1982). In general, the trends are not particularly clear in any of these studies. In efforts to characterize fish assemblages and assess the influence of stress and/or pollution, combinations of species diversity parameters and seasonal similarity in species compositions have been used (see Horn, 1980). T h e suggestion is that unstressed fish assemblages should show a high species diversity with low similarities in species composition between seasons. T h e combination of a very high diversity and the low levels of similarity between some months adds weight to the suggestion that this is an unstressed fish assemblage and that there is little human-induced pollution stress. T h e major changes in the species compositions in this assemblage occurred during the autumn and spring. In both cases there was an influx of juvenile P. bogaraveo. Tiffs species spawns between January and April in this region (Krug, 1990). T h e very large influx in April was probably due to the new O-group fish coming in to the nursery area. T h e smaller elevation in abundance in September represents the previous year's juveniles returning to the shallow embayments, possibly in an effort to avoid large pelagic predators. T h e peak in September was also caused by juvenile S. pilchardus in the bay. Juveniles are known to shoal in shallow water (Wheeler, 1969). Spawning generally occurs over the autumn and winter period (Whitehead et al., 1986). Therefore the influx of juveniles is possibly a method of avoiding the larger offshore pelagic predators (Wright, 1988). T h e other two dominant pelagic species in the bay (C. labrosus and T. ovatus) both have their highest abundance in July and August. These species are spring spawners (Wheeler, 1969) with the major recruitment of the new year class to the embayments occurring in the autumn. In the case of T. ovatus, there was a secondary arrival of young during the winter. Unfortunately there has been very little work done on these species in the Azores region, therefore m u c h of the general biology is unknown. T h e major benthic species were at a m u c h lower level of abundance than the dominant pelagics. T h e two carnivorous species appear to dominate the bay at different times. Dasyatis pastinaca was prevalent during the s u m m e r months, being replace by E. vipera for the rest of the year. Presumably there are very few fish species which would predate on either of these species. In the shallow coastal waters of south Africa, D. pastinaca is classed as a spring/summer resident (Lasiak, 1984b). T h e flatfish, B. podas, generally recruited to the bay in September as newly metamorphosing fish (see Nash et al., 1991). T h e numbers of juveniles declined rapidly but there were adults in the bay for most of the year. However, this species also occurs on hard substrata (Nash et al., 1991) and changes in abundance may be caused by movements between these two habitats. T o a certain extent, thee three species along with S. saurus and Sphoeroides marmoratus can be considered as residents while most of the others are migrant species. T h e y use the bay as either a nursery ground, as a rich feeding ground by predators (e.g. adult Pomatomus saltator, this species also occurs as juveniles in the bay), or as an occasional habitat for fishes from deep offshore waters (e.g. myctophids). We consider S. saurus may be a resident because it buries itself in the sand. Its low abundance in the seine samples is primarily due to its ability to bury. Individuals have been seen throughout the bay during diving studies (our observations). Sphoeroides marmoratus is also considered a resident because it appears to have a certain degree of fidelity to location and probably does not undertake large migrations out of the bay for spawning. In conclusion, there is a distinct seasonality to the community structure of the fish assemblage in Porto Pim Bay. T h e highest biomass occurs in the s u m m e r and this is
590
R. S. Santos & R. D. M. Nash
comparable to results obtained from other inshore marine environments. The fish f a u n a l diversity, w h e n c o m p a r e d w i t h a r e a s o n c o n t i n e n t s a n d t h e p r e s e n c e j u v e n i l e fish i n t h i s e n v i r o n m e n t , w h i c h is r a r e i n t h e A z o r e s , w a r r a n t s i n v e s t i g a t i o n s to u n d e r s t a n d t h e d y n a m i c s o f fish s p e c i e s a n d a s s e m b l a g e s i n
reduced of many further shallow
water in the middle of the Atlantic Ocean.
Acknowledgements T h e a u t h o r s are g r a t e f u l t o t h e B r i t i s h C o u n c i l ( P o r t u g a l ) , I N I C ( P o r t u g a l ) a n d JNICT/Ci~ncia (Portugal), the Azorean Regional Department of Fisheries (SRAP/DRP) and Azorean Department of the Environment (SRTA/DRA) for funding associated with this work. This project was initiated through the Department of Oceanography and Fisheries (University of the Azores)/Port Erin Marine Laboratory (University of L i v e r p o o l ) E x p e d i t i o n to t h e A z o r e s i n J u n e / J u l y 1 9 8 9 a n d o u r g r a t i t u d e is e x t e n d e d t o all s p o n s o r s o f t h a t e x p e d i t i o n . T h e a u t h o r s a c k n o w l e d g e t h e field a n d l a b o r a t o r y support given by Olavo Amaral, Vitor Rosa, Noberto Serpa, Fatima Serpa and Debbie Neves. Contribution No. 24 from the 'Expedition Azores 1989'.
References Ali, T. S. & Hussain, N. A. 1990 Composition and seasonal fluctuations of intertidal fish assemblage in Kohr al-Zubair, Northwestern Arabian Gulf. Journal of Applied Ichthyology 6, 24-36. Allen, L. G. 1982 Seasonal abundance, composition, and productivity of the littoral fish assemblage in upper Newport Bay, California. Fishery Bulletin 80, 769-790. Allen, L. G. & DeMartini, E. E. 1983 Temporal and spatial patterns of nearshore distribution and abundance of the pelagic fishes of San Onofre-Oceanside, California. Fishery Bulletin gl, 569-586. Allen, L. G., Horn, M. H., Edmonds, F. A. & Usui, C. A. 1983 Structure and seasonal dynamics of the fish assemblages in the Cabrillo beach area of Los Angeles harbor, California. Bulletin of the South California Acadamy of Science 82, 47-70. Bell, J. D., Pollard, D. A., Burchmore, J. J., Pease, B. C. & Middleton, M. J. 1984 Structure of a fish community in a temperate tidal mangrove creek in Botany Bay, New South Wales. Australian Journal of Marine and Freshwater Research 35, 33-46. Briggs, J. C. 1966 Oceanic islands, endemism and marine paleotemperatures. Systematic Zoology 15, 153-163. Briggs, J. C. 1970 A faunal history of the North Atlantic Ocean. Systematic Zoology 19, 19-34. Briggs, J. C. 1974 Marine Zoogeography. MacGraw-Hill, New York. Horn, M. H. 1980 Diel and seasonal variation in abundance and diversity of shallow-water fish populations in Morro Bay, California. Fishery Bullethz 78, 759-770. Krug, H. M. S. 1990 The Azorean blackspot seabream, Pagellus bogaraveo (Brtinnich, 1768) (Teleostei: Sparidae): Reproductive cycle, hermaphroditism, maturity and fecundity. Cybium 14, 151-159. Kubecka, J. & Bohm, M. 1991 The fish fauna of the Jordan reservoir, one of the oldest manmade lakes in Central Europe. Journal ofFish Biology 38, 935-950. Lasiak, T. A. 1984a Structural aspects of the surf-zone fish assemblage at King's beach, Algoa Bay, South Africa: Short-term fluctuations. Estuarine, Coastal and Shelf Science 18, 347-360. Lasiak, T. A. 1984b Structural aspects of the surf-zone fish assemblage at King's beach, Algoa Bay, South Africa: Long-term fluctuations. Estuarine, Coastal and Shelf Science 18, 459-483. Livingston, R. J. 1976 Diurnal and seasonal fluctuations of organisms in a north Florida estuary. Estuarine and Coastal Marine Science 4, 373-400. McCleave, J. C. & Fried, S. M. 1975. Nighttime catches of fishes in a tidal cove in Montsweag Bay near Wiscasset, Maine. Transactions of the American Fisheries Society 104, 153-163. McErlean, A. J., O'Connor, S. G., Milhursky, J. A. & Gibson, C. I. 1973 Abundance, diversity and seasonal patterns of estuarine fish populations. Estuarine and Coastal Marine Science 1, 19-36. Nash, R. D. M. 1988 The effects of disturbance and severe seasonal fluctuations in environmental conditions on north temperate shallow-water fish assemblages. Estuarine, Coastal and Shelf Science 26, 123-135. Nash, R. D. M. & Gibson, R. N. 1982 Seasonal fluctuations and compositions of two populations of small demersal fishes on the west coast of Scotland. Estuarine, Coastal and Shelf Science 15, 485-495.
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Nash, R. D. M. & Santos, R. S. 1993 The occurrence of the lesser weever in the Azores. Journal offish Biology 43, 317-319. Nash, R. D. M., Hawkins, S. J., Fisher, E. & Hill, A. 1990 Diel fluctuations and 15opulation structure of a sandy beach shallow-water fish assemblage. In Expediqao Aqores:89/Relatorio Preliminar (DOP/UA), Vol. II, (Santos, R. S., ed.). Universidade dos Aqores, pp. 233-250. Nash, R. D. M., Geffen, A. J. & Santos, R. S. 1991 The wide-eyed flounder, Bothus podas DeLaroche, a singular flatfish in varied shallow-water habitats of the Azores. Netherlands Journal of Sea Research 27, 367-373. Nash, R. D. M., Santos, R. S., Geffen, A. J., Hughes, G. & Ellis, T. R. 1994a Diel variability in catch rate of juvenile flatfish on two small nursery grounds (Port Erin Bay, Isle of Man and Porto Pim Bay, Faial, Azores). Journal ofFish Biology 44, 35-45. Nash, R. D. M., Santos, R. S. & Hawkins, S. J. 1994b Diel fluctuations of a sandy beach fish assemblage at Porto Pim, Azores. Arquip~lago-Life and Marine Science 12A, 75-86. Patzner, R. A., Santos, R. S., Rr, P. & Nash, R. D. M. 1992 Littoral fishes of the Azores: An annotated checklist of fishes observed during the "Expedition Azores 1989". Arquip~lago-Life and Earth Sciences 10, 101-111. Pielou, E. C. 1977 Mathematical Ecology. Wiley, New York. Quinn, N. J. 1980 Analysis of temporal changes in fish assemblages in Serpentine Creek, Queensland. Environmental Biology of Fishes 5, 117 133. Ross, S. T., McMichael, R. H. & Ruple, D. L. 1987 Seasonal and diel variation in the standing crop of fishes and macroinvertebrates from a Gulf of Mexico surf zone. Estuarine, Coastal and Shelf Science 25, 391-412. Santos, R. S., Nash, R. D. M. & Hawkins, S. J. 1994 Fish assemblages on intertidal shores of the island of Faial, Azores. Arquipklago-Life and Marine Sciences 12A, 87-100. Wheeler, A. 1969 The Fishes of the British Isles and North-west Europe. Macmillan, London. Whitehead, P. J. P., Bauchot, M.-L., Hureau, J.-C., Nielsen, J. & Tortonese, E. (eds.) 1.986 Fishes of the North-eastern Atlantic and Mediterranean, Vol. I-III. UNESCO, Chaucer Press, U.K. Whitfield, A. K. 1993 Fish biomass estimates from the littoral zone of an estuarine coastal lake. Estuaries 16, 280-289. Whittaker, R. H. & Fairbanks, C. W. 1958 A study of plankton copepod communities in the Columbian Basin, south-eastern Washington. Ecology 39, 46-65. Wright, J. M. 1988 Seasonal and spatial differences in the fish assemblage of the non-estuarine Sulaibikhat Bay, Kuwait. Marine Biology 100, 13-20. Wright, J. M. 1989 Diel variation and seasonal consistency in the fish assemblage of the non-estuarine Sulaibikhat Bay, Kuwait. Marine Biology 102, 135-142.