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On the ecology of the dragonet Callionymus lyra L. in the southern North Sea
139
Netherlands Journal of Sea Research 26 (1): 139-150 (1990)
ON THE ECOLOGY OF THE DRAGONET CALLIONYMUS LYRA L. IN THE SOUTHERN NORTH SEA* H.W. VAN DER VEER, F. CREUTZBERG, R. DAPPER, G.C.A. DUINEVELD, M. FONDS, B.R. KUIPERS, G.J. VAN NOORT and J.IJ. WlTTE Netherlands Institute for Sea Research, P.O. Box 59, 1790AB
ABSTRACT
Den Burg, Texel, The Netherlands
The present paper evaluates data collected in the southern North Sea between 1972 and 1984 on geographical distribution, density, growth, mortality, biomass, production and food requirements of the dragonet Callionymus lyra L. Catches never exceeded 1500 individuals per 10 000 m 2 and the dragonet showed a clear pattern of abundance with highest densities in the coastal zone and decreasing numbers going offshore. Mortality rates appeared to be rather constant over the age groups with z = 0.55 y-lo The dragonet population mainly consisted of 1, 2 and 3-year-old individuals with a mean length at the end of the year of 7.5, 15 and 19 cm, respectively. Production values were at least 159 g AFDW.10 000 m-2-y -1. Predation pressure exerted by the dragonets amounted to at least 795 g AFDWol0000 m-2.y -1. The correspondence between the von Bertalanffy growth curve in this study and that of 1948 (CHANG, 1951) suggests absence of effects of eutrophication in the area on the growth of dregonets between 1948 and the 1980s. The main difference with 1948 was the absence of large individuals, problably as a result of increased fishery in the area.
have already been published, such as predation by dab in the Oyster Ground area (DUlNEVELD & VAN NOORT, 1986) and the role of the lesser weever in the southern North Sea (CREUTZBERG & WITTE, 1989). This paper discusses the dragonet Callionymus lyra L. Dragonets are normally found in water less than 50 m deep, together with a related species, the reticulated dragonet C. reticulatus (WHEELER, 1969, 1978; BOER, 1971). The ecology of the reticulated dragonet will be discussed elsewhere. Dragonets spawn between February and April and both eggs and larvae are planktonic. After their larval development, they settle on the seabed. CHANG (1951) and WHEELER (1969, 1978) suggest a difference between the sexes in growth rate, maximum size and age: females are thought to live up to 7 years, reaching a maximum size of 20 cm, while males can reach 30 cm in 5 years. Though the 'Aurelia' cruises cover the relatively short period from 1972 to 1984, this time-series offers an opportunity to analyse changes in population structure and distribution that may be related with eutrophication and pollution of the area during the last decades. First, all data have been pooled for the estimation of the general role of the dragonet in the benthic system. Subsequently, possible long-term changes in the population structure are discussed.
1. INTRODUCTION
2. MATERIAL AND METHODS
Quantitative data are plentiful especially on commercially important epibenthic predators in the North Sea, but much less is known about the noncommercial ones. Besides, most studies concentrate on restricted areas. As a consequence, hardly any information is available on the role of a number of fish species in the ecosystem. The 'Aurelia' cruises carried out by the Netherlands Institute of Sea Research (NIOZ) in the 70s were aimed to fill in this gap in knowledge (CREUTZBERG, 1985), and they form an extensive data set covering especially the southern part of the North Sea. Data on some of the species
All hauls were made with a 5.5 m beam trawl with 2 tickler chains and a stretched mesh size in the codend of 10 mm. Haul duration was normally 10 min, and the distance covered was estimated with the Decca Track plots. Fig. 1 shows the positions of all hauls. Parts of the area were not covered in these surveys due to the presence of large stones. After sorting of the sample, all fishes (or a subsample) were counted and measured to the nearest 1.0 cm, 0.5 cm or 0.1 cm below, depending on the purpose. Callionymus lyra was distinguished from C. reticulatus by the absence of a spine at the base of the
*Publication no 15 of the project 'Applied Scientific Research Netherlands Institute for Sea Research (BEWON)'
140
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
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Fig. 1. Positions of beam-trawl hauls (0) made during the 'Aurelia' cruises from 1972to 1984, together with isobaths (m). preoperculum, according to WHEELER (1978). All numbers caught were converted into numbers per 10 000 m2 without correcting for net efficiency, since increasing the number of tickler chains had no effect on the catches of this species both in sandy and in muddy areas (CREUTZBERGetaL, 1985) and no information is available on other factors (lateral escape etc.). For age determination, individuals were deepfrozen and examined in the laboratory. Of each fish total length was determined in mm and age by means of otolith reading. Differences in day and night catches and lengthweight (L-W) relationships were determined in differ-
ent seasons at a fixed station (53°00'N; 3°55'E) from October 1978 to October 1979. Due to the small number of animals caught per survey, all individuals were pooled for the L-W relationship. Of all animals length in ram, wet weight in g and ash-free dry weight (AFDW) in g were determined. AFDW was estimated by desiccation for 2 days at 80°C and subsequent incineration of the samples at 560°C for 2 h. The weight loss at 560°C was considered to represent the AFDW. Food consumption of dragonets in relation to temperature was estimated under laboratory conditions at 15°C following the methods described in detail by
DRAGONET IN THE NORTH SEA
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Fig. 2. Distribution of Callionymus lyra (n.10 000 m -2) in the southern North Sea, divided into a number of subareas based on maximal occurrence per area.
FONDS et al. (1985), in combination with field observations on food intake and digestion at two other temperatures in October 1978 and May 1979. Stomach contents of dragonets were analysed in the field over a 24-h period. Hauls were made every 2 h. All fishes were measured in 0.5 cm total length classes. Stomach contents were preserved individually in a 4 % formalin-seawater solution and sorted out in the laboratory. All contents were expressed in AFDW, and related to the weight of the fish.
3. RESULTS 3.1. ABUNDANCE AND DISTRIBUTION To analyse the geographic distribution and density of the dragonet we used the method developed by CREUTZBERG & WITTE (1989), who applied it to the same material for the lesser weever. This method works primarily with maximum numbers. A plot of all data, irrespectively of time of the year, showed that the distribution of the dragonet was far
142
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
TABLE 1 Mean catches of Callionymus lyra (n.10 000 m -2) in the various sub-areas according to Fig. 2 sub area
density range
mean
s.d.
no of hauls
A B C D E F
> 1000 0-500 0-100 0- 50 0- 10 <10
923.3 48.4 20.9 10.9 3.3 1.3
369.8 55.6 22.8 11.6 3.2 2.7
3 226 154 83 17 15
from uniform in the southern North Sea. Maximum numbers found allowed the area to be divided into a number of subareas (Fig. 2). Table 1 shows the mean catches of Callionymus lyra for each subarea. Despite fluctuations in density, a general trend could be observed with highest numbers in the coastal zone and decreasing numbers towards deeper water offshore. The number of hauls per season was too low and the spatial variation in sampling too high to draw conclusions about possible seasonal migrations. Day and night catches differed significantly for most periods both in sandy and in silty areas (MannWhitney U-test, p<0.05; ELLIOT, 1971), but no general trend could be observed. The ratio between night and day catches changed with prevailing water temperature (Fig. 3). At water temperatures lower than 11°C, daytime catches exceeded night
catches, and at higher temperatures night catches were higher. These differences between day and night catches increased with increasing bottomwater temperature. 3.2. AGE DISTRIBUTION AND GROWTH The age distribution and estimates of growth were obtained in two ways: (a) The Petersen method, deciphering histograms of length frequencies collected from 1972 onwards. Fig. 4 shows the percentage size-frequency distributions of all dragonets caught per cruise, irrespective of sex. Only the 0-, I- and II-groups were clearly visible. The evident peaks in these population histograms were used to draw by eye tentative growth curves for 0-, I- and II- groups (Fig. 5). Growth of 0-, I- and II-groups seemed to be restricted to the period between May-June and October. Older age groups could not be traced.
mean total length (cm)
]I
night catches day catches 16
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1.2-
0.8-
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temperature (°C) Fig. 3. Ratio between night and day catches of Callionymus lyra L. in relation to water temperature in a silty (ll) and a sandy area (•).
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Fig. 5. Growth curves (cm) of the dragonet Callionymus lyra age groups 0 - III through the year, calculated from the length-frequency distribution according to the Petersen method: C) = 0; • = I; + = II; <> = Ill-group.
DRAGONET
IN T H E N O R T H
SEA
143
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8
12
16
20
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20 24 size closs (cm)
Fig. 4. Length-frequency distributions of all catches of Callionymus lyra from 1972 to 1981 in 1-cm total-length classes Indicated are the different age groups (by eye): © : 0-group; 0 : I-group; + : II-group; ~ : Ill-group
144
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
totel length cm) 2420 16.
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(]ge group Fig. 6. Growth of the dragonet
Callionymus lyra, based on otolith readings of a catch of
June 1989 (n = 100), separated for males (shaded) and females (white). The curve is computed after the von Bertalanfg/model for the data set of 1949 (CHANG,1951). (b) Otolith reading of individuals. The growth curve derived from the Petersen method was checked by otolith reading of a sample of about 100 individuals caught in June 1989, in which males and females were analysed separately (Fig. 6). In June 1989 not even a small 0-group was found, and all individuals belonged to I-, II- and partly to Ill-groups. No clear differences in size were found between males and females, and also the relationship between body length or standard length and total length was the same for both sexes (Table 2). Individuals 4, 5 and 6 years old were rare, which made it difficult to apply the von Bertalanffy growth model to obtain estimates of growth and maximal length:
model to the length-age distribution of June 1989 resulted in a reasonable fit of the curve with these data in the young age groups. Not enough older age groups could be collected to permit a statistical test between the two sets. However, at present there is no reason to suggest other parameters for the older age groups (Fig. 6). Therefore, the von Bertalanffy model has also been used for the present data. A comparison of the Petersen method with the results of the otolith reading showed a good correspondence between the results of the two methods, representing the situation around July. However, due to the few age groups present, no statistical test could be applied to support this suggestion.
Lt = Lmax(1-e- k(t - to)) in which Lt= length (cm) at age t, Lmax= maximal length (cm), k is growth rate ( y - t ) and to=the time that the 0-group is absent (y). Instead the data set on dragonets collected near Plymouth in 1948 and published by CHANG (1951) was used to compute these parameters (Table 3). Applying the resulting growth TABLE 2 Relationship between body or standard length (x) and total length (y) in Callionymus lyra for males and females, according to y=a,x a
males females
1.27 1.28
r
0.98 0.99
TABLE 3 Parameters of the von Bertalanffy model for Callionymus lyra as computed for the data set published by CHANG (1951) for the years 1948 - 1949, according to SPARRE(1987). Lmax in cm standard length.
Lrnax k to
parameter
unit
22.16 0.471 - 0.443
cm y- 1 y
DRAGONET IN THE NORTH SEA
145
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size class ( c m ) Fig. 7. Callionymus lyra. (a) Average population histogram in 1-cm classes based on all data between 1972 and 1981 expressed in relative abundance (%); age groups are indicated. (b) Constructed normal distributions per age group, based on Fig. 7a in combination with the von Bertalanffy growth model. 3.3. MORTALITY Sampling differed in stations and time of the year; therefore mean mortality for each year-class was estimated over the whole period of investigation, based on the mean age composition of the catches in combination with information on the length-age distribution estimated with the von Bertalanffy model according to CFIEUTZBERG & WITTE (1989). Fig. 7a shows the mean length composition based on all data over the period 1972 - 1981.0-group fish were present in low numbers only, caused by the low efficiency of the net for this size group. The mean size of the youngest age groups, as estimated by the Petersen method (0-, I-, II- and Ill-groups) is indicated in Fig. 7a, representing the situation in June. Assuming a normal distribution for each age group, also for the older ones, the size-frequency histogram can be split up starting with the I-group, according to the method of CREUTZBERG & WITTE (1989). In this way also the older age groups could be discriminated as indicated in Fig. 7b. Next, the relative abundance for each age group in % could be computed (Fig. 8).
relative abundance (%) 403020lO
86-
432-
age group Fig. 8. Survival curve for the age groups of dragonet Callionymus lyra in the southern North Sea based on relative abundance in the total catches (%), according to Fig. 7b.
146
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WlTTE
stomach content The resulting survival curve suggested that mortality rate was rather constant over the age groups, with a ( mg AFDW. g-l.met, wet weight ) mean value of z = 0.55 ( y - l ) .
m
./'~.
6-
3.4. BIOMASS AND PRODUCTION Biomass and production were estimated for the whole North Sea area, and for each subarea (see Fig. 2 and Table 1). For the calculation of biomass values, a mean length-wet weight relation was applied, ignoring the seasonal changes in lengthweight relationship (n = 33; r = 0.98):
42-
D~
•
e/
\e/
o
W = 0.0262 L 2.442 (g) in which L is total length in cm and W is wet weight in g. For the conversion of wet weight into dry weight, a factor of 0.20 was found (n = 33; r = 0.93) and from dry weight into AFDW a factor of 0.84 ( n = 3 3 r = 0.92). To relate food intake with biomass of the individul fish and with biomass of the whole population, body weights (g) should be converted into metabolic weight (Wmet) according to FONDS et al., 1985):
b
Wmet = W 0-8 (g)
O~O
Table 4 shows the average biomass values found in the subareas. For the whole southern North Sea 650 g wet weight per 10 000 m 2 was found. TABLE 4 Mean biomass (g wet weight.10 000 m -2) for the various subareas according to Fig. 2 and Table 1. sub area
densi~ range
mean biomass
A B C D E F
>1000 0- 500 0-100 0 - 50 0 - 10 < 10
13750 909 357 249 110 58
Two methods were applied to estimate production: (a) Food intake in relation to temperature. Food intake of Callionymus lyra was estimated from analysis of stomach contents for various size groups in the field and in the laboratory. Daily food consumption was expressed in g food intake per g metabolic wet weight of the fish. Stomach contents were analysed in the field in October 1978 and May 1979 (Fig. 9) and the total weight of all stomach contents were expressed as g AFDW per g metabolic weight. This metabolic weight of each haul was the sum of all individual metabolic weights. Stomach contents showed a clear periodicity with an increase during
6
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~
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~'6
2~
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~, time
Fig. 9. Stomach contents of dragonet (rag AFDW-g -1 metabolic wet weight) in the course of 24 h. Black bars indicate periods after sunset and before sunrise. • = muddy area; • = sandy area: a = May 1979; b = October 1978. the daytime. Food intake per day was estimated from the increase in stomach contents during the daytime, corrected for digestion as estimated from the decline in stomach contents during the night. This sum of increase in stomach content and digestion during that period gives an estimate of food intake in relation to the prevailing temperature in the field (9.5°C in May 1979 and 11.5°C in October 1978). In combination with an estimate of food consumption at 15°C in the laboratory these data showed an increase of food intake per g metabolic biomass of the dragonet in relation to temperature (Fig. 10). Combined data on food consumption, mean metabolic wet weight of the standard population of 348 g and temperature of the North Sea were used to estimate food intake of the population over the May September growing season (Table 5). Assuming a food conversion efficiency of 20% (according to the general discussion in FONDS et al. (1989)), this means a production of 106.6 g AFDW.10 000 m-2.y -1 In a similar way production can be estimated for the various subareas (Table 6).
DRAGONET IN THE NORTH SEA
TABLE 5 Food intake of Callionymus lyra (g AFDW.10 000 m -2) of the standard population (see text) during the growing season, according to Fig. 10.
food intake (mg A F D W . g -~ met. wet weight.d -1 )
25-
147
I / / /
Month
Temperature (°C)
May June July August
9.0 11.5 14.7 16.7
/
20"
t /
/
4
d •
15.
/
/ /
/
/
10.
.
TOTAL
0
I
I
4
86 129 133 185 533
..4
5-
0
Food intake (g AFDW)
8
I
12
I
16
I
20
t e m p e r a t u r e (°C) Fig. 10. Relationship between food intake (FI) (mg AFDW.d -1) per g metabolic wet weight and temperature (t) together with best-fitting curve: FI= 5.77.e°-°at (r = 0.93). For further information see text. (b) Allen method. This method (ALLEN, 1971) allows a quick estimate of production under various assumptions of growth and mortality. The ratio between mean biomass and production is related to the growth and mortality function and results in an estimate of the net production of the population in one year, i.e. the replacement of biomass. In Callionymus lyra growth could be described according to the von Bertalanffy growth model and mortality remained the same for all age groups as concluded from the exponential decrease in numbers over the age groups. Under these assumptions the ratio between production and biomass in a stable situation is given by the instantaneous mortality rate z (ALLEN, 1971). For the standard population with a biomass of 648 g wet weight and z = 0.55 ( y - l ) this would
mean a production of 356 g wet weight per 10 000 m 2 per year, or 60 g AFDW.10 000 m-2.y -1 To estimate the predation pressure exerted by the dragonet, stomach contents were analysed at 2 stations during 4 cruises in 1978 and 1979 (Table 7). Food items consisted mainly of echinoderms (up to 65%) both in sandy and silty areas. Combining these stomach content data with estimates of total food intake results in an estimated mean predation of 376 to 453 g AFDW.10 000 m-2-y -1 on echinoderms by dragonets in the southern North Sea (Table 8). Among other food sources, only worms appeared to be important: between 139 and 195 g AFDW.10 000 m -2. These figures are only related to the predation during the growing season and no information is available on the energy --and hence food intake-necessary for maintenance during the rest of the year. This means that the above figures are underestimations of the real predation pressure. 4. DISCUSSION All studies on the role of fish species as consumers in the ecosystem are complicated by the problems of catch efficiency of the net used. In this study, the magnitude of this problem was analysed in two ways. First, increasing the number of tickler chains in front of the net should lead to higher catches if specimens are buried in the sediment. However, CREUTZBERGet al. (1985) did not find any increase in catch with in
TABLE 6 Estimated food intake and production of Callionymus lyra (g AFDW.10 000 m-2) for the different subareas (Fig. 2).
Sub area A B C D E F
Metabolic biomass (g wet weight) 7595 486 192 130 53 26
Food intake (g AFDW)
Total (g AFDW)
May
June
July
August
1884 122 48 33 13 6
2427 181 71 48 20 9
2891 185 73 50 20 10
4038 258 102 69 28 14
11240 746 294 200 81 39
Production (g AFDW) 2248 149 59 40 16 8
148
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
TABLE 7 Mean food composition of stomach content of Callionymus lyra expressed as percentages of AFDW of the total food in all samples collected at a fixed sandy station (53°00'N, 3°55'E) and a muddy station during 4 cruises between 1978 and 1979. area
fish
crustaceans
worms
echinoderms
bivalves
unidentified
Sandy Silty
6.1
8.9 3.9
19.9 28.0
65.1 54.1
4.5
10.0
creasing numbers of tickler chains both in muddy and in sandy areas, suggesting that in dragonets this factor was of no importance. The second way was a comparison of day and night catches to estimate the effect of visual avoidance, a factor especially important for species using visual stimuli in catching prey. Since the stomach content analysis showed that feeding takes place mainly during the day, higher catches would be expected during the day, when the fish are active. However, the comparison of day and night catches over the year had such diverse results that only a correlation with water temperature could be found. At low temperatures, higher catches were found during the day, supporting the expectations. At higher temperatures, the picture was reversed, with increasing catches at night. This suggests a decreasing catchability at day temperatures increasing above 12°C. The most likely explanation is a more efficient escape behaviour. The maximum swimming speed for animals up to 50 cm will be about 10 times the body length according to BEAMISH (1978), which means in the range of 1 to 1.5 m ' s - 1. Compared with a fishing speed of about 1.5 to 2 m.s -1, this seems low, but it may be fast enough if the reaction sets in early, when the net is still some metres away. Problems concerning the catch efficiency of the net are furthermore complicated by the low abundance of 0-group dragonets in most of the catches. Since spawning occurs around April (WHEELER, 1978), 0-group dragonets will be absent in the catches until autumn. In June they can still be found in the plankton at a size of about 6 mm (van der Land, pers. comm.). The catch efficiency of the beam-trawl will be low due to escape through the meshes. 0-group dragonets seem to escape through TABLE 8 Predation pressure on the main food items of Callionymus lyra in the southern North Sea. Food item
Fish Crustaceans Vermes Echinodermata Mollusca
Predation pressure (g AFDW. 10 000 m -2.y- i)
0- 43 27- 62 139 - 195 377 - 453 0- 31
the meshes and larger individuals dart away from the opening of the net. The relation between net efficiency and temperature makes it very complicated to make proper estimates of abundance. Such estimates will be minimum estimates due to too low numbers of the 0-group and underestimation of the older dragonets in summer when temperatures are highest. For the assessment of subareas, the net efficiency has no effect, because the separation is based on the same catching methods. The results of this study partly contradict the data of 1948 in CHANG (1951). His data suggest a difference in growth rate between males and females and furthermore a different relation between body or standard length and total length. Such differences could not be traced in our data set; nor did growth rates differ. This might be due to the absence of older individuals in our data, since differences between males and females become visible with age. Reasons for the absence of these age groups in our catches are not known. A likely explanation is the increased fishery intensity since 1948, resulting in a shift towards younger age groups. DE VEEN (1971) mentioned an increase in relative abundance of dragonets along the Dutch coast between 1947 1957 and 1969 - 1970. However, absolute numbers and length measurements of these data sets are lacking at this moment. A comparison with 1948 is not possible due to lack of data on abundance, biomass and production for that period. The only parameter that can be compared is the growth rate according to the von Bertalanffy model. This appears to have remained unchanged between 1948 and the 70s, in spite of the eutrophication of the North Sea that has taken place especially in the coastal zone during the last decades (VAN DER VEER et al., 1989). To what extent this means a maximal growth rate of the dragonet in both periods cannot be concluded. At least there seems to be no impact of eutrophication on the growth of the dragonet over that period. Distribution patterns of Callionymus lyra in the southern North Sea generally correspond with information from other areas where the dragonet appears also to be rather common in the coastal zone (< 10 m) (LOPEZ-JAMAR, 1984; LE MAC, 1986). The general trend of decreasing numbers with increasing water depth corresponds with earlier observations (WHEEL-
DRAGONET IN THE NORTH SEA
ER, 1969, 1978). In close connection with the trend in abundance, biomass values varied. Mortality of dragonets appeared to be rather constant between ages. Specific predators of dragonets are unknown. WETSTEIJN (1982) mentioned the presence of dragonet specimens in stomachs of turbot Scophtha/mus maximus and the brill S. rhombus. Ca//ionymus/yra itself mainly preys upon echinoderms; in this study they constituted more than 50% of its stomach contents. WHEELER (1978) also mentioned crustaceans and polychaete worms as food items, but echinoderms were not listed at all. The importance of echinoderms in our study is only based on observations at two stations. At least, it seems that food composition may show large spatial variations. The total predation by dragonets is rather low in the southern North Sea (about 530 g AFDW.10 000 m-2), although this figure represents only predation during the growing season, omitting the requirements for maintenance during the rest of the year. These requirements can be estimated by combining the food intake in relation to temperature (Fig. 10) with the mean water temperature between 1 September and 1 May (about 10°C). This would mean a food intake of about 11 mg AFDW per g metabolic weight per day, or 919 g AFDW.10000 m2.y -1 during these 8 months. Although this figure is only a very rough estimate, it suggests that the requirements for maintenance in the period 1 September to 1 May are in the same order as the requirements for growth and maintenance from 1 May to 31 August. In work on the lesser weever in the same area, CREUTZBERG & WITTE (1989) estimate that the requirements for maintenance during the period September to May are 50% of those for growth. Their estimate, which seems to be more reliable and conservative, suggests that total predation of the dragonets is at least 530 + 500/0 of 530 = 795 g AFDW-10 000 m - 2 . y -1 In the same area, the predation pressure of the lesser weever can be estimated at about 378 g AFDW-10000m-2-y -1 (according to table 1 in CREUTZBERG & WITTE, 1989). This suggests that dragonets are more important as predators at least over the 1972 - 1980 period. The estimate of production, assuming an annual predation of 795 g AFDW. 10 000 m - 2 and a conversion efficiency of 20% (FONDS et al., 1989) resulted in values of 159 and minimally 60 g AFDW.10 000 m -2, depending on the method applied (the stomach content analysis or Allen method, respectively). Total production was estimated at minimally 159 g AFDW.10 000 m -2 based on stomach content analysis. On the other hand, in a stable population with a constant mortality rate such as dragonets (z = 0.55 y-1), the loss of biomass due to mortality must be compensated for by production. Based on
149
ALLEN (1971) this production can be estimated at 60 g AFDW.10 000 m -2 for the whole population per year. The differences between these two figures can be attributed to the difference between total production of the whole population over a year and population replenishment due to mortality only. The difference will mainly be caused by tissue loss due to the requirements for maintenance in winter, estimated at about 50 g AFDW.10 000 m-2-y -1, and by loss of energy by reproductive products. Moreover, due to the uncertainties in the methods applied, the above figures will only represent the order of magnitude of the various processes. 5. REFERENCES ALLEN, K.R., 1971. Relation between production and biomass.--J. Fish. Res. Bd Canada 28: 1573-1581. BOER, P., 1971. The occurrence of Callionymus reticulatus in the southern North Sea.--J. Cons. int. Explor. Mer 33: 506-509. BEAMISH, F.W.H., 1978. Swimming capacity. In: W.S. HOAR& D.J. RANDALL.Fish physiology. Vol VII: Locomotion. Acad. Press New York: 111-187. CHANG, R-W., 1951. Age and growth of Ca//ionymus/yra L.--J. mar. Biol. Ass. 30: 281-296. CREUTZBERG,F., 1985. 'Aurelia'-cruise reports of the benthic fauna of the southern North Sea. Introductory report.--Interne Verslagen Nederlands Instituut voor Onderzoek der Zee, Texel. 1979-4: 1-31. CREUTZBERG, F., G.C.A. DUINEVELD & G.J. VAN NOORT, 1985. 'Aurelia'-cruise reports of the benthic fauna of the southern North Sea. Report 10. The effect of different numbers of tickler chains on beam-trawl catches.--Interne Verslagen Nederlands Instituut voor Onderzoek der Zee, Texel. 1981-1: 1-27. CREUTZBERG, F. & J.IJ. WITTE, 1989. An attempt to estimate the predatory pressure exerted by the lesser weever, Trachinus vipera Cuvier, in the southern North Sea.--J. Fish Biol. 34: 429-449. DUINEVELD,G.C.A. & G.J. VAN NOORT, 1986. Observations on the population dynamics of Amphiura filiformis (Ophiuroidae: Echinodermata) in the southern North Sea and its exploitation by the dab, Limanda fimanda.--Neth J. Sea Res. 20" 85-94. ELLIOT, J.M., 1971. Some methods for the statistical analysis of samples of benthic invertebrates.--Sci. Publ. Freshwat. Biol. Ass. No 25: 1-160. FONDS, M., R. CRONIE, D. VETHAAK& P. VAN DER PUYL, 1985. Laboratory measurements of maximum daily food intake, growth and oxygen consumption of plaice (Pleuronectes platessa) and flounder (Paralichthys flesus) in relation to water temperature and size of the fish. ICES C.M. Dem. Fish Comm. 1985/G:54: 1-7. FONDS, M., B. DRINKWAARD,J.W. RENSINK,G.G.J. EYSINK & W. TOET, 1989. Laboratory measurements of metabolism, food intake and growth of Solea solea (L.) fed with mussel meat or with dry food. In: N. DE PAUW, E. JASPERS,H. ACKEFORS& N. WILKINS.Aquaculture - a biotechnology in progress. Europ. Aquac. Soc., Bredene, Belgium: 1-24.
150
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
LE MAO, P., 1986. Feeding relationships between the benthic infauna and the dominant benthic fish of the Rance estuary (France).-- J. mar. biol. Ass. U.K. 66: 391-401. LOPEZ-JAMAR, E., J. IGLESIAS & J.J. OTERO, 1984. Contribution of infauna and mussel-raft epifauna to demersal fish diets.--Mar. Ecol. Prog. Ser. 15: 13-18. SPARRE W.E., 1987. Computer programs for fish stock assessment. Length based fish stock assessment for Apple II computers.--FAO Fish. Tech. Pap. No 101 Suppl. 2. VEEN, J. DE, 1971. Veranderingen in de visstand in de Noordzee.--Waddenbulletin 6: 2-7.
VEER, H. W. VAN DER, W. VAN RAAPHORST & M.J.N. BERGMAN, 1989. Eutrophication of the Dutch Wadden Sea: external nutrient Ioadings of the Marsdiep and Vliestroom basin.--Helgol&nder Meeresunters. 43: 501-516. WETSTEIJN, L.P.M.J., 1982. Een onderzoek naar de voedselopname van tarbot (Scophthalmusmaximus L.) en griet (Scophtha/mus rhombus L.) in de zuidelijke Noordzee.--Neth. Inst. Fish. Invest. IJmuiden, Rep ZE 82-02: 1-44. WHEELER, A., 1969. The fishes of the British Isles and North-west Europe. Macmillan, London: 1-613. - - - - , 1978. Key to the fishes of Northern Europe. Frederick Warne, London: 1-380.
Netherlands Journal of Sea Research 26 (1): 139-150 (1990)
ON THE ECOLOGY OF THE DRAGONET CALLIONYMUS LYRA L. IN THE SOUTHERN NORTH SEA* H.W. VAN DER VEER, F. CREUTZBERG, R. DAPPER, G.C.A. DUINEVELD, M. FONDS, B.R. KUIPERS, G.J. VAN NOORT and J.IJ. WlTTE Netherlands Institute for Sea Research, P.O. Box 59, 1790AB
ABSTRACT
Den Burg, Texel, The Netherlands
The present paper evaluates data collected in the southern North Sea between 1972 and 1984 on geographical distribution, density, growth, mortality, biomass, production and food requirements of the dragonet Callionymus lyra L. Catches never exceeded 1500 individuals per 10 000 m 2 and the dragonet showed a clear pattern of abundance with highest densities in the coastal zone and decreasing numbers going offshore. Mortality rates appeared to be rather constant over the age groups with z = 0.55 y-lo The dragonet population mainly consisted of 1, 2 and 3-year-old individuals with a mean length at the end of the year of 7.5, 15 and 19 cm, respectively. Production values were at least 159 g AFDW.10 000 m-2-y -1. Predation pressure exerted by the dragonets amounted to at least 795 g AFDWol0000 m-2.y -1. The correspondence between the von Bertalanffy growth curve in this study and that of 1948 (CHANG, 1951) suggests absence of effects of eutrophication in the area on the growth of dregonets between 1948 and the 1980s. The main difference with 1948 was the absence of large individuals, problably as a result of increased fishery in the area.
have already been published, such as predation by dab in the Oyster Ground area (DUlNEVELD & VAN NOORT, 1986) and the role of the lesser weever in the southern North Sea (CREUTZBERG & WITTE, 1989). This paper discusses the dragonet Callionymus lyra L. Dragonets are normally found in water less than 50 m deep, together with a related species, the reticulated dragonet C. reticulatus (WHEELER, 1969, 1978; BOER, 1971). The ecology of the reticulated dragonet will be discussed elsewhere. Dragonets spawn between February and April and both eggs and larvae are planktonic. After their larval development, they settle on the seabed. CHANG (1951) and WHEELER (1969, 1978) suggest a difference between the sexes in growth rate, maximum size and age: females are thought to live up to 7 years, reaching a maximum size of 20 cm, while males can reach 30 cm in 5 years. Though the 'Aurelia' cruises cover the relatively short period from 1972 to 1984, this time-series offers an opportunity to analyse changes in population structure and distribution that may be related with eutrophication and pollution of the area during the last decades. First, all data have been pooled for the estimation of the general role of the dragonet in the benthic system. Subsequently, possible long-term changes in the population structure are discussed.
1. INTRODUCTION
2. MATERIAL AND METHODS
Quantitative data are plentiful especially on commercially important epibenthic predators in the North Sea, but much less is known about the noncommercial ones. Besides, most studies concentrate on restricted areas. As a consequence, hardly any information is available on the role of a number of fish species in the ecosystem. The 'Aurelia' cruises carried out by the Netherlands Institute of Sea Research (NIOZ) in the 70s were aimed to fill in this gap in knowledge (CREUTZBERG, 1985), and they form an extensive data set covering especially the southern part of the North Sea. Data on some of the species
All hauls were made with a 5.5 m beam trawl with 2 tickler chains and a stretched mesh size in the codend of 10 mm. Haul duration was normally 10 min, and the distance covered was estimated with the Decca Track plots. Fig. 1 shows the positions of all hauls. Parts of the area were not covered in these surveys due to the presence of large stones. After sorting of the sample, all fishes (or a subsample) were counted and measured to the nearest 1.0 cm, 0.5 cm or 0.1 cm below, depending on the purpose. Callionymus lyra was distinguished from C. reticulatus by the absence of a spine at the base of the
*Publication no 15 of the project 'Applied Scientific Research Netherlands Institute for Sea Research (BEWON)'
140
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
.
0 ~o"
~
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30"
,~
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.
.
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.
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,
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,
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"
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3o . . . . . . . . . .
2°
30"
£,
3 °
30"
-"
"~
4°
~o"
30"
5 °
30 ,
Fig. 1. Positions of beam-trawl hauls (0) made during the 'Aurelia' cruises from 1972to 1984, together with isobaths (m). preoperculum, according to WHEELER (1978). All numbers caught were converted into numbers per 10 000 m2 without correcting for net efficiency, since increasing the number of tickler chains had no effect on the catches of this species both in sandy and in muddy areas (CREUTZBERGetaL, 1985) and no information is available on other factors (lateral escape etc.). For age determination, individuals were deepfrozen and examined in the laboratory. Of each fish total length was determined in mm and age by means of otolith reading. Differences in day and night catches and lengthweight (L-W) relationships were determined in differ-
ent seasons at a fixed station (53°00'N; 3°55'E) from October 1978 to October 1979. Due to the small number of animals caught per survey, all individuals were pooled for the L-W relationship. Of all animals length in ram, wet weight in g and ash-free dry weight (AFDW) in g were determined. AFDW was estimated by desiccation for 2 days at 80°C and subsequent incineration of the samples at 560°C for 2 h. The weight loss at 560°C was considered to represent the AFDW. Food consumption of dragonets in relation to temperature was estimated under laboratory conditions at 15°C following the methods described in detail by
DRAGONET IN THE NORTH SEA
2°
30'
0
so
•
40
so"
141
5
0
so"
30
o
!
54
3O
I
t
53 °
3O
/
3o
2°
30"
3°
30"
o
30"
5o
30"
Fig. 2. Distribution of Callionymus lyra (n.10 000 m -2) in the southern North Sea, divided into a number of subareas based on maximal occurrence per area.
FONDS et al. (1985), in combination with field observations on food intake and digestion at two other temperatures in October 1978 and May 1979. Stomach contents of dragonets were analysed in the field over a 24-h period. Hauls were made every 2 h. All fishes were measured in 0.5 cm total length classes. Stomach contents were preserved individually in a 4 % formalin-seawater solution and sorted out in the laboratory. All contents were expressed in AFDW, and related to the weight of the fish.
3. RESULTS 3.1. ABUNDANCE AND DISTRIBUTION To analyse the geographic distribution and density of the dragonet we used the method developed by CREUTZBERG & WITTE (1989), who applied it to the same material for the lesser weever. This method works primarily with maximum numbers. A plot of all data, irrespectively of time of the year, showed that the distribution of the dragonet was far
142
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
TABLE 1 Mean catches of Callionymus lyra (n.10 000 m -2) in the various sub-areas according to Fig. 2 sub area
density range
mean
s.d.
no of hauls
A B C D E F
> 1000 0-500 0-100 0- 50 0- 10 <10
923.3 48.4 20.9 10.9 3.3 1.3
369.8 55.6 22.8 11.6 3.2 2.7
3 226 154 83 17 15
from uniform in the southern North Sea. Maximum numbers found allowed the area to be divided into a number of subareas (Fig. 2). Table 1 shows the mean catches of Callionymus lyra for each subarea. Despite fluctuations in density, a general trend could be observed with highest numbers in the coastal zone and decreasing numbers towards deeper water offshore. The number of hauls per season was too low and the spatial variation in sampling too high to draw conclusions about possible seasonal migrations. Day and night catches differed significantly for most periods both in sandy and in silty areas (MannWhitney U-test, p<0.05; ELLIOT, 1971), but no general trend could be observed. The ratio between night and day catches changed with prevailing water temperature (Fig. 3). At water temperatures lower than 11°C, daytime catches exceeded night
catches, and at higher temperatures night catches were higher. These differences between day and night catches increased with increasing bottomwater temperature. 3.2. AGE DISTRIBUTION AND GROWTH The age distribution and estimates of growth were obtained in two ways: (a) The Petersen method, deciphering histograms of length frequencies collected from 1972 onwards. Fig. 4 shows the percentage size-frequency distributions of all dragonets caught per cruise, irrespective of sex. Only the 0-, I- and II-groups were clearly visible. The evident peaks in these population histograms were used to draw by eye tentative growth curves for 0-, I- and II- groups (Fig. 5). Growth of 0-, I- and II-groups seemed to be restricted to the period between May-June and October. Older age groups could not be traced.
mean total length (cm)
]I
night catches day catches 16
2.0-
1.6-
÷
12 t •
/
1.2-
0.8-
I
0
4
temperature (°C) Fig. 3. Ratio between night and day catches of Callionymus lyra L. in relation to water temperature in a silty (ll) and a sandy area (•).
•
+
•
it
" d
0.4--
•
0
el|
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1
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I
I
I
M AM
O I
d
I
d
I
A
I
S
I
I
ON
I
Fig. 5. Growth curves (cm) of the dragonet Callionymus lyra age groups 0 - III through the year, calculated from the length-frequency distribution according to the Petersen method: C) = 0; • = I; + = II; <> = Ill-group.
DRAGONET
IN T H E N O R T H
SEA
143
%
% 10-' 10 0-
204
0¸
10
10 ¸
0
10
972 i38
O-
11979 =296
10
0 73 !9
10-
:t 1979 n=76
10
0
i ~
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i
i
i
i
i
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0Ctn=9481974 1980 = 215
10 0
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10-
n, i
0
,
i
i
i
i
4
8
12
16
20
24
0
4
8
12
16
20 24 size closs (cm)
Fig. 4. Length-frequency distributions of all catches of Callionymus lyra from 1972 to 1981 in 1-cm total-length classes Indicated are the different age groups (by eye): © : 0-group; 0 : I-group; + : II-group; ~ : Ill-group
144
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
totel length cm) 2420 16.
.f
D
///
128.
/ /
/
/
4-
I
I
I
~
I
I
I
~I
I~
Z
(]ge group Fig. 6. Growth of the dragonet
Callionymus lyra, based on otolith readings of a catch of
June 1989 (n = 100), separated for males (shaded) and females (white). The curve is computed after the von Bertalanfg/model for the data set of 1949 (CHANG,1951). (b) Otolith reading of individuals. The growth curve derived from the Petersen method was checked by otolith reading of a sample of about 100 individuals caught in June 1989, in which males and females were analysed separately (Fig. 6). In June 1989 not even a small 0-group was found, and all individuals belonged to I-, II- and partly to Ill-groups. No clear differences in size were found between males and females, and also the relationship between body length or standard length and total length was the same for both sexes (Table 2). Individuals 4, 5 and 6 years old were rare, which made it difficult to apply the von Bertalanffy growth model to obtain estimates of growth and maximal length:
model to the length-age distribution of June 1989 resulted in a reasonable fit of the curve with these data in the young age groups. Not enough older age groups could be collected to permit a statistical test between the two sets. However, at present there is no reason to suggest other parameters for the older age groups (Fig. 6). Therefore, the von Bertalanffy model has also been used for the present data. A comparison of the Petersen method with the results of the otolith reading showed a good correspondence between the results of the two methods, representing the situation around July. However, due to the few age groups present, no statistical test could be applied to support this suggestion.
Lt = Lmax(1-e- k(t - to)) in which Lt= length (cm) at age t, Lmax= maximal length (cm), k is growth rate ( y - t ) and to=the time that the 0-group is absent (y). Instead the data set on dragonets collected near Plymouth in 1948 and published by CHANG (1951) was used to compute these parameters (Table 3). Applying the resulting growth TABLE 2 Relationship between body or standard length (x) and total length (y) in Callionymus lyra for males and females, according to y=a,x a
males females
1.27 1.28
r
0.98 0.99
TABLE 3 Parameters of the von Bertalanffy model for Callionymus lyra as computed for the data set published by CHANG (1951) for the years 1948 - 1949, according to SPARRE(1987). Lmax in cm standard length.
Lrnax k to
parameter
unit
22.16 0.471 - 0.443
cm y- 1 y
DRAGONET IN THE NORTH SEA
145
%
o-4
_
I I
0
l
Z
IKZ~
10
0
0
4
8
12
16
20
24
size class ( c m ) Fig. 7. Callionymus lyra. (a) Average population histogram in 1-cm classes based on all data between 1972 and 1981 expressed in relative abundance (%); age groups are indicated. (b) Constructed normal distributions per age group, based on Fig. 7a in combination with the von Bertalanffy growth model. 3.3. MORTALITY Sampling differed in stations and time of the year; therefore mean mortality for each year-class was estimated over the whole period of investigation, based on the mean age composition of the catches in combination with information on the length-age distribution estimated with the von Bertalanffy model according to CFIEUTZBERG & WITTE (1989). Fig. 7a shows the mean length composition based on all data over the period 1972 - 1981.0-group fish were present in low numbers only, caused by the low efficiency of the net for this size group. The mean size of the youngest age groups, as estimated by the Petersen method (0-, I-, II- and Ill-groups) is indicated in Fig. 7a, representing the situation in June. Assuming a normal distribution for each age group, also for the older ones, the size-frequency histogram can be split up starting with the I-group, according to the method of CREUTZBERG & WITTE (1989). In this way also the older age groups could be discriminated as indicated in Fig. 7b. Next, the relative abundance for each age group in % could be computed (Fig. 8).
relative abundance (%) 403020lO
86-
432-
age group Fig. 8. Survival curve for the age groups of dragonet Callionymus lyra in the southern North Sea based on relative abundance in the total catches (%), according to Fig. 7b.
146
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WlTTE
stomach content The resulting survival curve suggested that mortality rate was rather constant over the age groups, with a ( mg AFDW. g-l.met, wet weight ) mean value of z = 0.55 ( y - l ) .
m
./'~.
6-
3.4. BIOMASS AND PRODUCTION Biomass and production were estimated for the whole North Sea area, and for each subarea (see Fig. 2 and Table 1). For the calculation of biomass values, a mean length-wet weight relation was applied, ignoring the seasonal changes in lengthweight relationship (n = 33; r = 0.98):
42-
D~
•
e/
\e/
o
W = 0.0262 L 2.442 (g) in which L is total length in cm and W is wet weight in g. For the conversion of wet weight into dry weight, a factor of 0.20 was found (n = 33; r = 0.93) and from dry weight into AFDW a factor of 0.84 ( n = 3 3 r = 0.92). To relate food intake with biomass of the individul fish and with biomass of the whole population, body weights (g) should be converted into metabolic weight (Wmet) according to FONDS et al., 1985):
b
Wmet = W 0-8 (g)
O~O
Table 4 shows the average biomass values found in the subareas. For the whole southern North Sea 650 g wet weight per 10 000 m 2 was found. TABLE 4 Mean biomass (g wet weight.10 000 m -2) for the various subareas according to Fig. 2 and Table 1. sub area
densi~ range
mean biomass
A B C D E F
>1000 0- 500 0-100 0 - 50 0 - 10 < 10
13750 909 357 249 110 58
Two methods were applied to estimate production: (a) Food intake in relation to temperature. Food intake of Callionymus lyra was estimated from analysis of stomach contents for various size groups in the field and in the laboratory. Daily food consumption was expressed in g food intake per g metabolic wet weight of the fish. Stomach contents were analysed in the field in October 1978 and May 1979 (Fig. 9) and the total weight of all stomach contents were expressed as g AFDW per g metabolic weight. This metabolic weight of each haul was the sum of all individual metabolic weights. Stomach contents showed a clear periodicity with an increase during
6
4
~
1'2
~'6
2~
•
~, time
Fig. 9. Stomach contents of dragonet (rag AFDW-g -1 metabolic wet weight) in the course of 24 h. Black bars indicate periods after sunset and before sunrise. • = muddy area; • = sandy area: a = May 1979; b = October 1978. the daytime. Food intake per day was estimated from the increase in stomach contents during the daytime, corrected for digestion as estimated from the decline in stomach contents during the night. This sum of increase in stomach content and digestion during that period gives an estimate of food intake in relation to the prevailing temperature in the field (9.5°C in May 1979 and 11.5°C in October 1978). In combination with an estimate of food consumption at 15°C in the laboratory these data showed an increase of food intake per g metabolic biomass of the dragonet in relation to temperature (Fig. 10). Combined data on food consumption, mean metabolic wet weight of the standard population of 348 g and temperature of the North Sea were used to estimate food intake of the population over the May September growing season (Table 5). Assuming a food conversion efficiency of 20% (according to the general discussion in FONDS et al. (1989)), this means a production of 106.6 g AFDW.10 000 m-2.y -1 In a similar way production can be estimated for the various subareas (Table 6).
DRAGONET IN THE NORTH SEA
TABLE 5 Food intake of Callionymus lyra (g AFDW.10 000 m -2) of the standard population (see text) during the growing season, according to Fig. 10.
food intake (mg A F D W . g -~ met. wet weight.d -1 )
25-
147
I / / /
Month
Temperature (°C)
May June July August
9.0 11.5 14.7 16.7
/
20"
t /
/
4
d •
15.
/
/ /
/
/
10.
.
TOTAL
0
I
I
4
86 129 133 185 533
..4
5-
0
Food intake (g AFDW)
8
I
12
I
16
I
20
t e m p e r a t u r e (°C) Fig. 10. Relationship between food intake (FI) (mg AFDW.d -1) per g metabolic wet weight and temperature (t) together with best-fitting curve: FI= 5.77.e°-°at (r = 0.93). For further information see text. (b) Allen method. This method (ALLEN, 1971) allows a quick estimate of production under various assumptions of growth and mortality. The ratio between mean biomass and production is related to the growth and mortality function and results in an estimate of the net production of the population in one year, i.e. the replacement of biomass. In Callionymus lyra growth could be described according to the von Bertalanffy growth model and mortality remained the same for all age groups as concluded from the exponential decrease in numbers over the age groups. Under these assumptions the ratio between production and biomass in a stable situation is given by the instantaneous mortality rate z (ALLEN, 1971). For the standard population with a biomass of 648 g wet weight and z = 0.55 ( y - l ) this would
mean a production of 356 g wet weight per 10 000 m 2 per year, or 60 g AFDW.10 000 m-2.y -1 To estimate the predation pressure exerted by the dragonet, stomach contents were analysed at 2 stations during 4 cruises in 1978 and 1979 (Table 7). Food items consisted mainly of echinoderms (up to 65%) both in sandy and silty areas. Combining these stomach content data with estimates of total food intake results in an estimated mean predation of 376 to 453 g AFDW.10 000 m-2-y -1 on echinoderms by dragonets in the southern North Sea (Table 8). Among other food sources, only worms appeared to be important: between 139 and 195 g AFDW.10 000 m -2. These figures are only related to the predation during the growing season and no information is available on the energy --and hence food intake-necessary for maintenance during the rest of the year. This means that the above figures are underestimations of the real predation pressure. 4. DISCUSSION All studies on the role of fish species as consumers in the ecosystem are complicated by the problems of catch efficiency of the net used. In this study, the magnitude of this problem was analysed in two ways. First, increasing the number of tickler chains in front of the net should lead to higher catches if specimens are buried in the sediment. However, CREUTZBERGet al. (1985) did not find any increase in catch with in
TABLE 6 Estimated food intake and production of Callionymus lyra (g AFDW.10 000 m-2) for the different subareas (Fig. 2).
Sub area A B C D E F
Metabolic biomass (g wet weight) 7595 486 192 130 53 26
Food intake (g AFDW)
Total (g AFDW)
May
June
July
August
1884 122 48 33 13 6
2427 181 71 48 20 9
2891 185 73 50 20 10
4038 258 102 69 28 14
11240 746 294 200 81 39
Production (g AFDW) 2248 149 59 40 16 8
148
VAN DER VEER, CREUTZBERG, DAPPER, DUINEVELD, KUIPERS, FONDS, VAN NOORT & WITTE
TABLE 7 Mean food composition of stomach content of Callionymus lyra expressed as percentages of AFDW of the total food in all samples collected at a fixed sandy station (53°00'N, 3°55'E) and a muddy station during 4 cruises between 1978 and 1979. area
fish
crustaceans
worms
echinoderms
bivalves
unidentified
Sandy Silty
6.1
8.9 3.9
19.9 28.0
65.1 54.1
4.5
10.0
creasing numbers of tickler chains both in muddy and in sandy areas, suggesting that in dragonets this factor was of no importance. The second way was a comparison of day and night catches to estimate the effect of visual avoidance, a factor especially important for species using visual stimuli in catching prey. Since the stomach content analysis showed that feeding takes place mainly during the day, higher catches would be expected during the day, when the fish are active. However, the comparison of day and night catches over the year had such diverse results that only a correlation with water temperature could be found. At low temperatures, higher catches were found during the day, supporting the expectations. At higher temperatures, the picture was reversed, with increasing catches at night. This suggests a decreasing catchability at day temperatures increasing above 12°C. The most likely explanation is a more efficient escape behaviour. The maximum swimming speed for animals up to 50 cm will be about 10 times the body length according to BEAMISH (1978), which means in the range of 1 to 1.5 m ' s - 1. Compared with a fishing speed of about 1.5 to 2 m.s -1, this seems low, but it may be fast enough if the reaction sets in early, when the net is still some metres away. Problems concerning the catch efficiency of the net are furthermore complicated by the low abundance of 0-group dragonets in most of the catches. Since spawning occurs around April (WHEELER, 1978), 0-group dragonets will be absent in the catches until autumn. In June they can still be found in the plankton at a size of about 6 mm (van der Land, pers. comm.). The catch efficiency of the beam-trawl will be low due to escape through the meshes. 0-group dragonets seem to escape through TABLE 8 Predation pressure on the main food items of Callionymus lyra in the southern North Sea. Food item
Fish Crustaceans Vermes Echinodermata Mollusca
Predation pressure (g AFDW. 10 000 m -2.y- i)
0- 43 27- 62 139 - 195 377 - 453 0- 31
the meshes and larger individuals dart away from the opening of the net. The relation between net efficiency and temperature makes it very complicated to make proper estimates of abundance. Such estimates will be minimum estimates due to too low numbers of the 0-group and underestimation of the older dragonets in summer when temperatures are highest. For the assessment of subareas, the net efficiency has no effect, because the separation is based on the same catching methods. The results of this study partly contradict the data of 1948 in CHANG (1951). His data suggest a difference in growth rate between males and females and furthermore a different relation between body or standard length and total length. Such differences could not be traced in our data set; nor did growth rates differ. This might be due to the absence of older individuals in our data, since differences between males and females become visible with age. Reasons for the absence of these age groups in our catches are not known. A likely explanation is the increased fishery intensity since 1948, resulting in a shift towards younger age groups. DE VEEN (1971) mentioned an increase in relative abundance of dragonets along the Dutch coast between 1947 1957 and 1969 - 1970. However, absolute numbers and length measurements of these data sets are lacking at this moment. A comparison with 1948 is not possible due to lack of data on abundance, biomass and production for that period. The only parameter that can be compared is the growth rate according to the von Bertalanffy model. This appears to have remained unchanged between 1948 and the 70s, in spite of the eutrophication of the North Sea that has taken place especially in the coastal zone during the last decades (VAN DER VEER et al., 1989). To what extent this means a maximal growth rate of the dragonet in both periods cannot be concluded. At least there seems to be no impact of eutrophication on the growth of the dragonet over that period. Distribution patterns of Callionymus lyra in the southern North Sea generally correspond with information from other areas where the dragonet appears also to be rather common in the coastal zone (< 10 m) (LOPEZ-JAMAR, 1984; LE MAC, 1986). The general trend of decreasing numbers with increasing water depth corresponds with earlier observations (WHEEL-
DRAGONET IN THE NORTH SEA
ER, 1969, 1978). In close connection with the trend in abundance, biomass values varied. Mortality of dragonets appeared to be rather constant between ages. Specific predators of dragonets are unknown. WETSTEIJN (1982) mentioned the presence of dragonet specimens in stomachs of turbot Scophtha/mus maximus and the brill S. rhombus. Ca//ionymus/yra itself mainly preys upon echinoderms; in this study they constituted more than 50% of its stomach contents. WHEELER (1978) also mentioned crustaceans and polychaete worms as food items, but echinoderms were not listed at all. The importance of echinoderms in our study is only based on observations at two stations. At least, it seems that food composition may show large spatial variations. The total predation by dragonets is rather low in the southern North Sea (about 530 g AFDW.10 000 m-2), although this figure represents only predation during the growing season, omitting the requirements for maintenance during the rest of the year. These requirements can be estimated by combining the food intake in relation to temperature (Fig. 10) with the mean water temperature between 1 September and 1 May (about 10°C). This would mean a food intake of about 11 mg AFDW per g metabolic weight per day, or 919 g AFDW.10000 m2.y -1 during these 8 months. Although this figure is only a very rough estimate, it suggests that the requirements for maintenance in the period 1 September to 1 May are in the same order as the requirements for growth and maintenance from 1 May to 31 August. In work on the lesser weever in the same area, CREUTZBERG & WITTE (1989) estimate that the requirements for maintenance during the period September to May are 50% of those for growth. Their estimate, which seems to be more reliable and conservative, suggests that total predation of the dragonets is at least 530 + 500/0 of 530 = 795 g AFDW-10 000 m - 2 . y -1 In the same area, the predation pressure of the lesser weever can be estimated at about 378 g AFDW-10000m-2-y -1 (according to table 1 in CREUTZBERG & WITTE, 1989). This suggests that dragonets are more important as predators at least over the 1972 - 1980 period. The estimate of production, assuming an annual predation of 795 g AFDW. 10 000 m - 2 and a conversion efficiency of 20% (FONDS et al., 1989) resulted in values of 159 and minimally 60 g AFDW.10 000 m -2, depending on the method applied (the stomach content analysis or Allen method, respectively). Total production was estimated at minimally 159 g AFDW.10 000 m -2 based on stomach content analysis. On the other hand, in a stable population with a constant mortality rate such as dragonets (z = 0.55 y-1), the loss of biomass due to mortality must be compensated for by production. Based on
149
ALLEN (1971) this production can be estimated at 60 g AFDW.10 000 m -2 for the whole population per year. The differences between these two figures can be attributed to the difference between total production of the whole population over a year and population replenishment due to mortality only. The difference will mainly be caused by tissue loss due to the requirements for maintenance in winter, estimated at about 50 g AFDW.10 000 m-2-y -1, and by loss of energy by reproductive products. Moreover, due to the uncertainties in the methods applied, the above figures will only represent the order of magnitude of the various processes. 5. REFERENCES ALLEN, K.R., 1971. Relation between production and biomass.--J. Fish. Res. Bd Canada 28: 1573-1581. BOER, P., 1971. The occurrence of Callionymus reticulatus in the southern North Sea.--J. Cons. int. Explor. Mer 33: 506-509. BEAMISH, F.W.H., 1978. Swimming capacity. In: W.S. HOAR& D.J. RANDALL.Fish physiology. Vol VII: Locomotion. Acad. Press New York: 111-187. CHANG, R-W., 1951. Age and growth of Ca//ionymus/yra L.--J. mar. Biol. Ass. 30: 281-296. CREUTZBERG,F., 1985. 'Aurelia'-cruise reports of the benthic fauna of the southern North Sea. Introductory report.--Interne Verslagen Nederlands Instituut voor Onderzoek der Zee, Texel. 1979-4: 1-31. CREUTZBERG, F., G.C.A. DUINEVELD & G.J. VAN NOORT, 1985. 'Aurelia'-cruise reports of the benthic fauna of the southern North Sea. Report 10. The effect of different numbers of tickler chains on beam-trawl catches.--Interne Verslagen Nederlands Instituut voor Onderzoek der Zee, Texel. 1981-1: 1-27. CREUTZBERG, F. & J.IJ. WITTE, 1989. An attempt to estimate the predatory pressure exerted by the lesser weever, Trachinus vipera Cuvier, in the southern North Sea.--J. Fish Biol. 34: 429-449. DUINEVELD,G.C.A. & G.J. VAN NOORT, 1986. Observations on the population dynamics of Amphiura filiformis (Ophiuroidae: Echinodermata) in the southern North Sea and its exploitation by the dab, Limanda fimanda.--Neth J. Sea Res. 20" 85-94. ELLIOT, J.M., 1971. Some methods for the statistical analysis of samples of benthic invertebrates.--Sci. Publ. Freshwat. Biol. Ass. No 25: 1-160. FONDS, M., R. CRONIE, D. VETHAAK& P. VAN DER PUYL, 1985. Laboratory measurements of maximum daily food intake, growth and oxygen consumption of plaice (Pleuronectes platessa) and flounder (Paralichthys flesus) in relation to water temperature and size of the fish. ICES C.M. Dem. Fish Comm. 1985/G:54: 1-7. FONDS, M., B. DRINKWAARD,J.W. RENSINK,G.G.J. EYSINK & W. TOET, 1989. Laboratory measurements of metabolism, food intake and growth of Solea solea (L.) fed with mussel meat or with dry food. In: N. DE PAUW, E. JASPERS,H. ACKEFORS& N. WILKINS.Aquaculture - a biotechnology in progress. Europ. Aquac. Soc., Bredene, Belgium: 1-24.
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LE MAO, P., 1986. Feeding relationships between the benthic infauna and the dominant benthic fish of the Rance estuary (France).-- J. mar. biol. Ass. U.K. 66: 391-401. LOPEZ-JAMAR, E., J. IGLESIAS & J.J. OTERO, 1984. Contribution of infauna and mussel-raft epifauna to demersal fish diets.--Mar. Ecol. Prog. Ser. 15: 13-18. SPARRE W.E., 1987. Computer programs for fish stock assessment. Length based fish stock assessment for Apple II computers.--FAO Fish. Tech. Pap. No 101 Suppl. 2. VEEN, J. DE, 1971. Veranderingen in de visstand in de Noordzee.--Waddenbulletin 6: 2-7.
VEER, H. W. VAN DER, W. VAN RAAPHORST & M.J.N. BERGMAN, 1989. Eutrophication of the Dutch Wadden Sea: external nutrient Ioadings of the Marsdiep and Vliestroom basin.--Helgol&nder Meeresunters. 43: 501-516. WETSTEIJN, L.P.M.J., 1982. Een onderzoek naar de voedselopname van tarbot (Scophthalmusmaximus L.) en griet (Scophtha/mus rhombus L.) in de zuidelijke Noordzee.--Neth. Inst. Fish. Invest. IJmuiden, Rep ZE 82-02: 1-44. WHEELER, A., 1969. The fishes of the British Isles and North-west Europe. Macmillan, London: 1-613. - - - - , 1978. Key to the fishes of Northern Europe. Frederick Warne, London: 1-380.