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The influence of nursery ground processes in the determination of year-class strength in juvenile plaice Pleuronectes platessa L. in Port Erin Bay, Irish Sea
Journal of Sea Research 44 (2000) 101±110
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The in¯uence of nursery ground processes in the determination of year-class strength in juvenile plaice Pleuronectes platessa L. in Port Erin Bay, Irish Sea R.D.M. Nash*, A.J. Geffen Port Erin Marine Laboratory, School of Biological Sciences, University of Liverpool, Port Erin, Isle of Man IM9 6JA, UK Received 24 September 1999; accepted 15 April 2000
Abstract The interannual variability in settlement and mortality of juvenile plaice Pleuronectes platessa L. was investigated between 1992 and 1998 on Port Erin Bay, west side of the Isle of Man, Irish Sea. The dampening in¯uence of factors operating on the nursery grounds was especially obvious in 1992 and 1996. In these years extremely high numbers of individuals settled, yet the population sizes in July were similar to other years. Thus the nursery ground processes were likely to be density dependent. Shrimp and crab densities were low in Port Erin Bay and probably had little predatory impact on young plaice. Crustacean densities were not signi®cantly related to winter temperatures. In the Irish Sea, year-class strength is determined during the nursery ground phase, in contrast to the North Sea where determination of year-class strength occurs prior to the nursery ground phase. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Irish Sea; plaice; Pleuronectes platessa; juvenile; settlement; growth; density-dependent mortality; population
1. Introduction Variations in recruitment of the European plaice, Pleuronectes platessa L., have been attributed to different processes acting on early survival. The young stages of plaice successively occupy two different environments. As eggs and larvae, plaice occupy pelagic environments whereas juveniles occur in demersal habitats in the sandy beach areas that they use as nursery grounds. It has been suggested that interannual variability in plaice recruitment is generated in the pelagic phase (Zijlstra and Witte, 1985; Van der Veer, 1986) and the nursery ground phase dampens variability. The majority of the * Corresponding author. E-mail address: [email protected] (R.D.M. Nash).
research on juvenile plaice has concentrated on North Sea stocks and thus most ideas concerning juvenile plaice population dynamics have been tested in the North Sea. Fish populations in different areas can exhibit different strategies (Nash et al., 2000), and the mechanisms determining year-class strength in North Sea populations may not be the same as for other areas. The majority of research on juvenile plaice on nursery grounds comes from a limited number of locations, the largest of which is the Dutch Wadden Sea (e.g. Kuipers, 1977; Zijlstra et al., 1982; Van der Veer, 1986; Van der Veer and Bergman, 1986, 1987; Van der Veer et al., 1990, 1994, 1997). The processes which operate on Wadden Sea nursery grounds may be distinctly different from those operating on nursery grounds around the British Isles (Van der Veer et al.,
1385-1101/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 1385-110 1(00)00044-7
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R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
1990). The largest data sets from the British Isles come from Filey Bay on the northeast coast of England (Lockwood, 1974), the west coast of Scotland (Edwards and Steele, 1968; Gibson, 1973; Gibson et al., 1993, 1996; Gibson and Robb, 1996) and the Irish Sea (Bowers, 1963; Jones and Kain, 1964; Riley and Corlett, 1965; Colman, 1966; Alhossaini et al., 1989; Nash et al., 1992; Nash et al., 1994a,b; Dau, 1994; Ellis, 1994; Hyder and Nash, 1998; S. De la Rosa, unpublished MSc thesis, University of Geneva). In both the North and Irish Seas, plaice spawn in the early part of the year (January±April) (Simpson, 1959a,b) and newly metamorphosed individuals settle in April to July onto sandy bays which serve as nursery grounds (Riley and Corlett, 1965; Colman, 1966; Hyder and Nash, 1998). At metamorphosis, plaice move from a relatively dilute, three-dimensional environment to a relatively concentrated twodimensional environment. Since there is a marked increase in density after settlement there has been considerable effort spent on determining the in¯uence of density-dependence in mortality during the nursery ground phase (Lockwood, 1980; Zijlstra et al., 1982; Van der Veer, 1986; Iles and Beverton, 1991; Beverton and Iles, 1992a,b). Both shrimp (Crangon crangon) and crabs (Carcinus maenas) have been implicated as major predators of young plaice (Van der Veer and Bergman, 1987). Although observations consistent with density-dependent effects have been noted, actual causal relationships have yet to be demonstrated. Port Erin Bay is a small (450 m wide) sandy bay on the south-west corner of the Isle of Man in the Irish Sea. This small bay has been the subject of many studies, particularly of the juvenile plaice population (see above). Much of the work has considered juvenile plaice in isolation from the rest of the assemblages present in the bay. In this paper we examine variation in instantaneous mortality rate both within the nursery ground phase and between years. We critically evaluate the hypotheses concerning the regulatory in¯uence of shrimp and crab populations on plaice nursery ground survival. Lastly, we determine at what point in the life cycle of plaice year-class strength is determined.
2. Methods 2.1. Juvenile numbers in Port Erin Bay Population sizes of juvenile plaice in Port Erin Bay for 1991 were taken from Ellis (1994). From 1991± 1998 ®ve transects were sampled in Port Erin Bay with a 1.5 m beam trawl between April and July and then a 2 m beam trawl between July and November (Fig. 1). The 1.5 m beam trawl had a 5 mm stretch mesh throughout and the 2 m beam trawl had 9 mm stretched mesh throughout. The gear was changed to accommodate the seasonal changes in ®sh size to avoid net clogging from accumulating vegetation. Sampling intensity varied between the years, from fortnightly in 1991±1992 to weekly (when weather permitted) between 1993 and 1998. Each transect was sampled once on each date. Both beam trawls were ®tted with triple tickler chains. All captured ®sh, C. crangon and crab species were counted. Plaice (total length) and C. crangon (carapace length) were measured to the nearest 1 mm. The abundance of plaice was calculated, for each transect separately, by multiplying the estimated density by the area of the bay which was sampled and known to be occupied by O-group plaice (101 000 m 2) (see Fig. 1). The total population size was estimated from the mean of all ®ve transects with its associated standard deviation. Ef®ciencies of the nets were assumed to be 20% prior to June and 30% thereafter (Edwards and Steele, 1968). These are the only published ef®ciencies applicable to this site. C. crangon and crab densities during plaice settlement were estimated as the mean density between May and July each year. Sea-surface temperature data for Port Erin Bay were collected daily and supplied by T. Shammon (Port Erin Marine Laboratory, Isle of Man). 2.2. Mortality rates Daily instantaneous mortality rates were calculated from: M loge Nt =N0 =Dt where M is the instantaneous daily mortality rate, Nt is the number at time t, N0 is the number at time 0 and Dt is the time period in days.
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110 55
SCOTLAND
IRELAND
ISLE OF MAN
54
ENGLAND
IRISH SEA
53
103
on the beach, and these are the normally quoted nursery ground mortality rates for juvenile plaice (Beverton and Iles, 1992a,b). We calculated an early mortality rate (EMR), covering the early part of the season, to account for the mortality that occurs during the settlement period, when the ®sh are potentially more vulnerable. The EMRs were calculated from the difference between the maximum population size and the total number of newly settled individuals: EMR loge Nmax =Nns =Dt
WALES
-6
-5
-4
T5
-3
where Nmax is the maximum number of individuals, Nns the sum of newly settled individuals between May and July, de®ned as ®sh ,17 mm (Nash and Geffen, 1999), and Dt constitutes the number of days between the date of 50% settlement (based on the cumulative percentage of ®sh ,17 mm), and the date of maximum population size. 2.3. Recruitment index of plaice in the Irish Sea
LW
Port Erin Bay HW T1
200m
Fig. 1. Port Erin Bay, Isle of Man, Irish Sea. The ®ve transects (T1± T5) sampled between 1992 and 1998 are shown. LW low water, HW high water. Area of the bay used to calculate population size is bounded by the dotted line and the low water mark (LW).
Nursery ground mortality was calculated for the settlement period (approximately April±June) in addition to the standard mortality estimate. The standard mortality rates (SMR) were calculated from the slope of the decline in population size over time from the date of the maximum population size (usually July) through to either October or November each year. The SMR estimates only include mortality that occurs after the majority of the ®sh have settled
The relationship between the nursery ground plaice populations and estimates of recruitment was examined to evaluate the role of nursery ground processes in determining year-class strength in the Irish Sea. Recruitment was de®ned as the mean catch of 1year-old plaice in the Irish Sea from the RV Corystes (CEFAS, Lowestoft) beam trawl surveys. Data on catches of 1-year-old plaice were obtained from the CEFAS September beam trawl series (RV Corystes). These data are also reported to the ICES Northern Shelf Demersal Working Group (ICES, 1998). Using the age-1 catch data rather than Virtual Population Analysis output for 1-year-old plaice abundance avoids bias due to the analytical technique used to estimate population size. 3. Results 3.1. Juvenile numbers in Port Erin Bay The population size of juvenile plaice in Port Erin Bay ¯uctuated by one order of magnitude between 1991 and 1998 (Fig. 2). The lowest population size was recorded in 1993 (maximum of approximately 40 000 individuals or a density of approximately 0.4 m 22) and the largest in 1996 (maximum of
104
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
Fig. 2. Variation in population size of O-group plaice, Pleuronectes platessa, in Port Erin Bay, Isle of Man between January 1991 and December 1998. Error bars 1 standard deviation.
304 300 individuals or a density of approximately 3.0 m 22).
There was a signi®cant positive relationship between the numbers of plaice that settle (Nns) and the subsequent EMR (Fig. 3a).
3.2. Mortality rates
EMR 5:68 £ 1025 £ Nns £ 1023 1 8:4 £ 1024
3.2.1. Early mortality rates (EMR) The EMR varied considerably between 1991 and 1998. The lowest early instantaneous daily mortality rate occurred in 1998 and the highest in 1996 (Table 1).
F 1;7 151:034; P , 0:01: The two years with very large settlement (1992 and 1996) had the highest instantaneous mortality rates.
Table 1 Estimated instantaneous mortalities of juvenile plaice in Port Erin Bay, Isle of Man, Irish Sea. EMR is estimated from the total number of settling ®sh. SMR is estimated from the decrease in juvenile abundance from the time of maximum population size through to the autumn Year
Early mortality rate (EMR)
Standard mortality rate (SMR)
Instantaneous mortality (d 21)
Instantaneous mortality (d 21)
R2
P
0.0318 0.0770 0.0248 0.0309 0.0207 0.0862 0.0234 0.0172
0.0161 0.0132 0.0235 0.0123 0.0108 0.0160 0.0238 0.0290 0.0212 0.0100 0.0160
0.97 0.98 0.95 0.917 0.835 0.879 0.783 0.953 0.929 0.754 0.932
, 0.05 , 0.05 , 0.01 , 0.01 . 0.05 , 0.01 , 0.01 , 0.01 , 0.01 , 0.01 . 0.05
a
1963 1964 a 1965 a 1991 1992 1993 1994 1995 1996 1997 1998 a
Data from Iles and Beverton (1991).
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
a
0.10
105
1993±1995, 1997±1998 where:
Early mortality rate (EMR; d-1)
EMR 25:9 £ 1023 £ Feb: temp 1 6:84 £ 1022 0.08
1992
1996
0.06
0.04
1991 1997
0.02
1994 1993 1995
1998 0.00 0
400
800 1200 Settlement Index (Nns)
1600
0.10
Early mortality rate (EMR; d-1)
b 1996
0.08
1992
0.06
0.04 1994 1991
1997 1993
0.02
0.00 6 7 8 9 Average February water temperature (oC)
3.2.2. Standard nursery ground mortality rates (SMR) Standard daily instantaneous mortality rates in Port Erin Bay ranged from 0.010 d 21 for the 1997 yearclass to 0.029 d 21 for the 1995 year-class (Table 1). There was a similar range for data collected for the 1963-1965 year-classes in Port Erin Bay. Density-dependent mortality during the later part of the nursery ground phase was examined by regressing the maximum estimated population size (after settlement was completed) against the SMR (Fig. 4a). There was no relationship between the maximum population size and the subsequent nursery ground mortality F 1;7 1:938; P . 0:05: The SMR was independent F 1;10 0:325; P . 0:05 of mean temperatures between May and August (Fig. 4b). There was no trend in the relationship between the February water temperature and the SMR F 1;7 0:018; P . 0:05 (Fig. 4c). The inclusion of data generated by Riley and Corlett (1965) did not improve the relationship F 1;10 0:005; P . 0:05: 3.3. Predation by shrimp and crabs
1998 1995
5
F 1;5 96:10; P , 0:01:
10
Fig. 3. (a) Relationship between the EMR (instantaneous) and the numbers of juvenile O-group plaice that settle in Port Erin Bay. (b) Relationship between the EMR (instantaneous) of plaice in Port Erin Bay and the preceding February sea-surface water temperature (8C).
The relationship between EMR and February water temperature described two data groups; years with high mortality rates and years with low mortality rates. In general, EMR decreased with increasing February water temperature (Fig. 3b). There was a signi®cant inverse relationship between February water temperature and EMR for the years 1991,
Shrimp and crab densities in Port Erin Bay varied between years (shrimp CV: 29%; crab CV: 65%). Shrimps were approximately an order of magnitude more abundant than crabs. The variability in instantaneous mortality rates of plaice over these years (1995±1998) was 93% (CV) for EMR and 42% (CV) for SMR. There were no signi®cant relationships between the density of shrimps or crabs and either estimate of plaice instantaneous mortality rates. The density of C. crangon . 17 mm TL (i.e. those thought to be capable of predating young plaice, length frequencies of C. crangon can be found in Oh et al., 1999) was at most 0.1 m 22 (1995), a maximum of approximately 0.04 m 22 in 1996 and 1998 and very low (0.01 m 22) in 1997. These compare with maximum densities of settling plaice over the same time period of approximately 1.5 m 22 (1995), 3.5 m 22 in 1996 and 0.75 m 22 for 1997 and
106
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110 0.032
a
Standard mortality rate (SMR; d-1)
1995 0.028
0.024
1994 1996
0.020
1993
0.016
1998 1991 0.012
1992
1997
1998. The highest density of all size classes of C. crangon was approximately 1.8 m 22 in August 1995. Over the same time period, the highest density of crabs (Carcinus maenas, Liocarcinus puber, L. depurator, L. holsatus combined) was 0.05 m 22 in August 1996. There were no signi®cant relationships between shrimp or crab densities and the February water temperature F 1;3 1:59; 2.07; P . 0:05 for both) (Fig. 5(a) and (b)). There was, however, a signi®cant inverse relationship between mean shrimp and crab densities F 1;3 37:47; P , 0:05 (Fig. 5c).
0.008
0
100000
200000
300000
400000
Maximum population size
0.032
b
Standard mortality rate (SMR; d-1)
1995 0.028
1994
0.024
1965
1996 0.020
1998
1993
1963
0.016
1964 1991
0.012
1997
1992 0.008
10.8
11.2
11
12.0
12.4
12.8
13.2
Average May-August water temperature (oc)
0.032
c
1995 Standard mortality rate (SMR; d-1)
0.028
1994
0.024
1965
1996
3.4. Year-class strength There was no relationship between the numbers of plaice that settle in Port Erin Bay (at 1±4 months of age) and the numbers of 1-year-old plaice estimated from young ®sh surveys F 1;7 0:464; P . 0:05: Similarly there was no signi®cant relationship between the maximum population size (which can occur between May and August) seen in Port Erin Bay and the subsequent age-1 population F 1;7 2:820; P . 0:05: However, there was a signi®cant relationship between the population size in July (not always the maximum population size) in Port Erin Bay and the subsequent population size at age-1 F 1;6 10:186; P , 0:05 (Fig. 6a). There was a signi®cant relationship between the numbers of juveniles which settled and the maximum population size F 1;7 6:226; P , 0:05 (Fig. 6b) but not with the population size in July F 1;7 0:540; P . 0:05 (Fig. 6c). This suggests that the early part of the nursery ground phase is critical in determining yearclass strength. Further evidence for year-class strength being ®xed during the nursery ground phase comes from analysis of the interannual variability in population size at different stages. There was relatively little variation
0.020
1993
1963
1998
0.016
1964
1991 0.012
1992 1997
0.008
4
5
6
7
8
9
Average February water temperature (oC)
10
Fig. 4. (a) The relationship between SMR (instantaneous) and the maximum annual O-group plaice population size in Port Erin Bay. (b) Relationship between the SMR (instantaneous) of plaice in Port Erin Bay and the ambient sea-surface temperature between May and August in Port Erin Bay. (c) Relationship between the SMR (instantaneous) and the average February sea-surface temperature. Circles data collected in this study, closed circles signi®cant relationships to calculate mortality rates, open circles non-signi®cant relationships, crosses data from Iles and Beverton (1991).
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
a
0.40
Crangon density (ind m-2)
0.36
1998 0.32
1997
0.28
107
(CV 20%) in the estimated Spawning Stock Biomass (SSB) in the Irish Sea between 1990 and 1998. Over this time period, the numbers of ®sh settling in Port Erin Bay and the maximum population sizes were highly variable (CV 62 and 58%, respectively). However, the interannual variation (CV) in population size at age-1 was only 29%, close to the level observed for the adult population.
0.24
4. Discussion
1996
0.20
1995 0.16 6.8
7.2
7.6
8.0
8.4
8.8
9.2
February water temperature (oC) 0.016
b
1996 1995
Crab density (ind m-2)
0.012
0.008
1997 0.004
1998 0.000 6.8
7.2
7.6
8.0
8.4
8.8
9.2
February water temperature (oC) 0.016
c
1996 1995
Crab density (ind m-2)
0.012
0.008
1997 0.004
Interannual variability in population size is common in juvenile plaice populations (Bergman et al., 1988; Van der Veer et al., 1990; Modin and Pihl, 1994) and Port Erin Bay is no exception. The population sizes recorded between 1991 and 1998 are similar to those observed in the 1960s (Jones and Kain, 1964; Riley and Corlett, 1965). Maximum densities of Ogroup plaice in Port Erin Bay ranged from 0.4 m 22 in 1993 to 3.0 m 22 in 1996. The maximum densities and the range are comparable to the range and magnitude recorded for the Balgzand tidal ¯ats of the Dutch Wadden Sea (Bergman et al., 1988) suggesting that similar density-related processes may occur in both areas. The maximum densities recorded for both the Wadden Sea and Port Erin Bay are lower than some recorded for a small Swedish bay (Gullmar Bay, 1.3± 10.0 ind. m 22, Iles and Beverton, 1991; Modin and Pihl, 1994), comparable to Ardmucknish Bay (west coast of Scotland, 1.5±3.5 ind. m 22, Iles and Beverton, 1991) and higher than Filey Bay (east coast of England, 1.2 ind. m 22, Iles and Beverton, 1991). Densities are generally mean values integrated over the horizontal distance of a beach area and depth. Spatial differences in abundance (e.g. Lockwood, 1974; Riley et al., 1986) imply that the density experienced by individuals may be higher and lower than the reported mean values. The daily instantaneous mortality rates recorded for Port Erin Bay (0.011±0.029 d 21) are within the range reported in the literature for a number of other nursery grounds (0.007±0.052 d 21; Iles and Beverton, 1991).
1998 0.000 0.16
0.20
0.24 0.28 0.32 Crangon density (ind m-2)
0.36
0.40
Fig. 5. Relationships between the average February sea surface temperature in Port Erin Bay and the mean density of: (a) Crangon crangon; (b) crabs; and (c) the relationship between C. crangon and crab densities.
108
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110 14
a
Recruitment index (Irish Sea)
1995
12 1996
10 1997
1994
1992 1991
8 1993
6 0
40000
80000
120000
160000
Population size in July (n) 400000
Maximum population size (Nmax)
b
300000
1996
200000
1995 1994 100000
1997 1991 1998
1993
1992
0
0 160000
400 800 1200 Settlement index (Nns)
1600
c Population size in July (n)
1995 1996
120000
1994
80000
1997 1991
40000
1993
1992
1998 0 0
Apart from the two mortality estimates of Alhossaini et al. (1989) for Red Wharf Bay in 1986 and 1987 it appears that mortality on Irish Sea nursery grounds is within the range shown at Port Erin. Daily instantaneous mortality also varies between years. In Port Erin Bay, mortality rates may be more in¯uenced by density-dependent factors since elevated mortalities were observed at higher densities. There was only a weak relationship between juvenile plaice survival and winter temperatures in Port Erin Bay. In the Wadden Sea cold winter/spring conditions reduce the shrimp and crab populations and the predation on plaice is subsequently low (Van der Veer et al., 1990; Ansell and Gibson, 1993). In Port Erin Bay there was no relationship between the February temperature and instantaneous mortality, nor was there a relationship between temperature and the density of shrimp. Whether this is indicative of a different predation mechanism working in open `British bays' as compared with the Wadden Sea or some other factor is unknown. Shrimp and crab predation in Port Erin Bay have a much smaller effect on young plaice dynamics than may occur in other areas. C. crangon densities recorded between 1995 and 1998 were very low (maximum 1.8 m 22) when compared with Ardmucknish Bay in 1990 (maximum 6.6 m 22, Ansell et al., 1999) and the long-term average (22 years) for 15 stations in the Wadden Sea (approximately 60 m 22, Beukema, 1992). Similarly the maximum crab densities in Port Erin Bay (0.05 m 22) were considerably lower than in Ardmucknish Bay (0.88 m 22, Ansell et al., 1999). Oh et al. (2000) examined the stomachs of shrimp in Port Erin Bay for the period 1995±1998 and showed that approximately 18±20% of large shrimps had ®sh remains in their stomachs. The incidence of young plaice (10±15 mm) in shrimp stomachs is higher in the Wadden Sea (Van der Veer et al., 1994, 1997) and may be a consequence of higher shrimp densities there. Fish predators have some impact on juvenile plaice (Gibson and Robb, 1996). In Port Erin Bay (1995±
400 800 1200 Settlement index (Nns)
1600
Fig. 6. Relationships between: (a) the recruitment index for age-1 plaice in the Irish Sea and plaice population size in Port Erin Bay in July; (b) the maximum recorded population size in Port Erin Bay and the settlement index (Nns); and (c) the O-group plaice population size in Port Erin Bay in July and the settlement index (Nns).
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
1998) there were young plaice in the stomachs of whiting (Merlangius merlangus), cod (Gadus morhua), saithe (Pollachius virens), pollack (P. pollachius), poor cod (Trisopterus minutus), ®ve-bearded rockling (Ciliata mustela) and I 2 group plaice (P. platessa) (S. De la Rosa, unpublished MSc thesis, University of Geneva). The predators were primarily I 1 group ®sh. The impacts of this predation and potential predation by birds (see Van der Veer et al., 1997) have not, however, been quanti®ed. Thus the evidence available suggests that density-dependent mortality may be due to predation (perhaps by ®sh) and food availability may play an important role. The year-class strength in Irish Sea plaice is probably not ®xed until at least July and indications of year-class strengths are measurable on the nursery grounds. If the small Port Erin Bay population is indicative of nursery ground relationships for the Irish Sea as a whole, then interannual variability is generated primarily at stage-I egg production (Nash and Geffen, 1999) and it is not until into the nursery ground stage that the interannual variation decreases. The decrease in interannual variability indicated that year-class strength is ®xed during the nursery ground phase. In the North Sea year-class strength is measurable by the time of settlement, indicating that it is ®xed during the pelagic stages (Zijlstra and Witte, 1985; Van der Veer, 1986; Van der Veer et al., 1997). The dampening in¯uence of the nursery grounds, possibly due to density-dependent effects, was especially obvious in 1992 and 1996. In these years extremely high numbers of individuals settled, yet the population sizes in July were similar to other years. Port Erin Bay may be a good example of a nursery ground ®lter, where carrying capacity leads to density-dependent survival and the determination of year-class strength. In summary, the population dynamics of Port Erin Bay have many similarities to the dynamics of the Wadden Sea. In general, the mortality rates are higher than in the Wadden Sea but there is a certain degree of uniformity within the Irish Sea as a whole (especially for the smaller nursery grounds). The number of juvenile plaice which survive to the end of the ®rst year (and hence become the basis of year-class strength) is a function of the numbers present and the mortality rates. Therefore, an examination of mortality rates alone will not give insights into year-
109
class strength. Data from Port Erin Bay suggest that good year-classes can come from lower than expected mortality rates rather than low mortality rates. Acknowledgements We are grateful for the help many people have given during the sampling from 1990 to present, especially to Graham Hughes for the work undertaken over the whole time period reported here and Cheryl Corkill for the work undertaken between 1995 and 1998. We are grateful to T.R. Ellis (NERC CASE student Ð University of Liverpool and Dunstaffnage Marine Laboratory) and K. Dau (MSc student Ð University of Geneva, ®eld work with the University of Liverpool) for permission to use their Port Erin Bay plaice data. Data on settlement for 1991 were obtained during the tenure of an NERC grant (GR9/185) to RDMN and AJG and the data from 1995 to early 1997 were obtained during the tenure of an NERC grant (GST/02/902) to RDMN and from DML, Robin Gibson and Mike Burrows. References Alhossaini, M., Liu, Q., Pitcher, T.J., 1989. Otolith microstructure indicating growth and mortality among plaice, Pleuronectes platessa L., post-larval sub-cohorts. J. Fish Biol. 35 (Suppl. A), 81±90. Ansell, A.D., Gibson, R.N., 1993. The effect of sand and light on predation of juvenile plaice (Pleuronectes platessa) by ®shes and crustaceans. J. Fish Biol. 43, 837±845. Ansell, A.D., Comely, C.A., Robb, L., 1999. Distribution, movements and diet of macrocrustaceans on a Scottish sandy beach with particular reference to predation on juvenile ®shes. Mar. Ecol. Prog. Ser. 176, 115±130. Bergman, M.J.N., Van der Veer, H.W., Zijlstra, J.J., 1988. Plaice nurseries: effect on recruitment. J. Fish Biol. 33 (Suppl. A), 201±218. Beukema, J.J., 1992. Dynamics of juvenile shrimp Crangon crangon in a tidal-¯at nursery of the Wadden Sea after mild and cold winters. Mar. Ecol. Prog. Ser. 83, 157±165. Beverton, R.J.H., Iles, T.C., 1992a. Mortality rates of O-group plaice (Pleuronectes platessa L.), dab (Limanda limanda L.) and turbot (Scophthalmus maximus L.) in European waters. II. Comparison of mortality rates and construction of life table for O-group plaice. Neth. J. Sea Res. 29, 49±59. Beverton, R.J.H., Iles, T.C., 1992b. Mortality rates of O-group plaice (Pleuronectes platessa L.), dab (Limanda limanda L.) and turbot (Scophthalmus maximus L.) in European waters. III. Density-dependence of mortality rates of O-group plaice
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and some demographic implications. Neth. J. Sea Res. 29, 61± 79. Bowers, A.B., 1963. Growth of young plaice in Port Erin Bay. Ann. Rep. Biol. Port Erin. 75, 28±32. Colman, J.A., 1966. Studies on the biology of young plaice in Manx waters, PhD thesis, University of Liverpool. Dau, K., 1994. Population dynamics of O-group plaice, Pleuronectes platessa L., on a Manx nursery ground, including the study of sub-cohorts by use of otolith microstructure, MSc thesis, University of Geneva. Edwards, R.R.C., Steele, J.H., 1968. The ecology of O-group plaice and common dabs in Loch Ewe. I. Population and food. J. Exp. Mar. Biol. Ecol. 2, 215±238. Ellis, T.R., 1994. Production and mortality of early life stages of ¯at®shes, PhD thesis, University of Liverpool. Gibson, R.N., 1973. The intertidal movements and distribution of young ®sh on a sandy beach with special reference to the plaice (Pleuronectes platessa). J. Exp. Mar. Biol. Ecol. 12, 79±102. Gibson, R.N., Robb, L., 1996. Piscine predation on juvenile ®shes on a Scottish sandy beach. J. Fish Biol. 49, 120±138. Gibson, R.N., Ansell, A.D., Robb, L., 1993. Seasonal and annual variations in abundance and species composition of ®sh and macrocrustacean communities on a Scottish sandy beach. Mar. Ecol. Prog. Ser. 98, 89±105. Gibson, R.N., Robb, L., Burrows, M.T., Ansell, A.D., 1996. Tidal, diel and longer term changes in the distribution of ®shes on a Scottish sandy beach. Mar. Ecol. Prog. Ser. 130, 1±17. Hyder, K., Nash, R.D.M., 1998. Variations in settlement pattern of Irish Sea plaice (Pleuronectes platessa L.) as determined from a simulation model. J. Sea Res. 40, 59±71. ICES, 1998. Report of the Working Group on the Assessment of Northern Shelf Demersal Stocks, ICES CM 1999/ACFM:1 545pp. Iles, T.C., Beverton, R.J.H., 1991. Mortality rates of O-group plaice (Pleuronectes platessa L.), dab (Limanda limanda L.) and turbot (Scophthalmus maximus L.) in European waters. 1. Statistical analysis of the data and estimation of the parameters. Neth. J. Sea Res. 27, 217±235. Jones, N.S., Kain, J.M., 1964. The numbers of O-group plaice in Port Erin Bay in August 1963 observed by diving. Rep. Mar. Biol. Sta. Port Erin 76, 19±25. Kuipers, B.R., 1977. On the ecology of juvenile plaice on a tidal ¯at in the Wadden Sea. Neth. J. Sea Res. 11, 56±91. Lockwood, S.J., 1974. The settlement, distribution and movements of O-group plaice Pleuronectes platessa (L.) in Filey Bay, Yorkshire. J. Fish Biol. 6, 465±477. Lockwood, S.J., 1980. Density-dependent mortality in O-group plaice (Pleuronectes platessa L.) populations. J. Cons. 39, 148±153. Modin, J., Pihl, L., 1994. Differences in growth and mortality of juvenile plaice, Pleuronectes platessa L., following normal and extremely high settlement. Neth. J. Sea Res. 32, 331±341. Nash, R.D.M., Geffen, A.J., 1999. Variability in stage 1 plaice, Pleuronectes platessa, egg production on the west side of the Isle of Man, Irish Sea. Mar. Ecol. Prog. Ser. 189, 241±250. Nash, R.D.M., Geffen, A.J., Hughes, G., 1992. Winter growth of juvenile plaice in Port Erin Bay. J. Fish Biol. 41, 209±215. Nash, R.D.M., Santos, R.S., Geffen, A.J., Hughes, G., Ellis, T.,
1994a. Diel variability in catch rate of juvenile ¯at®sh on two small nursery grounds (Port Erin Bay, Isle of Man and Porto Pim, Faial, Azores). J. Fish Biol. 44, 35±45. Nash, R.D.M., Geffen, A.J., Hughes, G., 1994b. Individual growth of juvenile plaice on an Irish Sea nursery ground (Port Erin Bay, Isle of Man). Neth. J. Sea Res. 32, 369±378. Nash, R.D.M., Witthames, P.R., Pawson, M., Alesworth, E., 2000. Regional variability in the dynamics of reproduction and growth of Irish Sea plaice, Pleuronectes platessa L. J. Sea Res. 44, 55± 64 (this issue). Oh, C.W., Hartnoll, R.G., Nash, R.D.M., 1999. Population dynamics of the common shrimp (Crangon crangon L.) in Port Erin Bay, Isle of Man, Irish Sea. ICES. J. Mar. Sci. 56, 718±733. Oh, C.W., Hartnoll, R.G., Nash, R.D.M., 2000. Feeding ecology of the common shrimp (Crangon crangon L.) in Port Erin Bay, Isle of Man, Irish Sea. Mar. Ecol. Prog. Ser., in press. Riley, J.D., Corlett, J., 1965. The numbers of O-group plaice in Port Erin Bay, 1964±66. Rep. Mar. Biol. Sta. Port Erin 78, 51±56. Riley, J.D., Symonds, D.J., Woolner, L.E., 1986. Determination of the distribution of the planktonic and small demersal stages of ®sh in coastal waters of England and Wales and adjacent areas between 1970 and 1984. MAFF, Fish. Tech. Rep. 84, 1±23. Simpson, A.C., 1959a. The spawning of plaice (Pleuronectes platessa) in the Irish Sea. Fish. Invest. Lond. Ser. II 22 (8), 1±30. Simpson, A.C., 1959b. The spawning of plaice (Pleuronectes platessa) in the North Sea. Fish. Invest. Lond. Ser. II 22 (7), 1±111. Van der Veer, H.W., 1986. Immigration, settlement and densitydependent mortality of a larval and early post-larval plaice (Pleuronectes platessa L.) population in the western Wadden Sea. Mar. Ecol. Prog. Ser. 29, 223±236. Van der Veer, H.W., Bergman, M.J.N., 1986. Development of tidally related behaviour of a newly settled O-group plaice (Pleuronectes platessa) population in the western Wadden Sea. Mar. Ecol. Prog. Ser. 31, 121±129. Van der Veer, H.W., Bergman, M.J.N., 1987. Predation by crustaceans on a newly-settled O-group plaice (Pleuronectes platessa L.) population in the western Wadden Sea. Mar. Ecol. Prog. Ser. 35, 203±215. Van der Veer, H.W., Pihl, L., Bergman, M.J.N., 1990. Recruitment mechanisms in North Sea plaice, Pleuronectes platessa. Mar. Ecol. Prog. Ser. 64, 1±12. Van der Veer, H.W., Berghahn, R., Rijnsdorp, A.D., 1994. Impact of juvenile growth on recruitment in ¯at®sh. Neth. J. Sea Res. 32, 153±173. Van der Veer, H.W., Ellis, T., Miller, J.M., Pihl, L., Rijnsdorp, A.D., 1997. Size selective predation on juvenile North Sea ¯at®sh and possible implications for recruitment. In: Chambers, R.C., Trippel, E.A. (Eds.). Early Life History and Recruitment in Fish Populations. Chapman and Hall, London, pp. 279±303. Zijlstra, J.J., Witte, J.IJ., 1985. On the recruitment of O-group plaice in the North Sea. Neth. J. Zool. 35, 360±376. Zijlstra, J.J., Dapper, R., Witte, J.IJ., 1982. Settlement, growth and mortality of post-larval plaice (Pleuronectes platessa) in the western Wadden Sea. Neth. J. Sea Res. 15, 250±272.
www.elsevier.nl/locate/seares
The in¯uence of nursery ground processes in the determination of year-class strength in juvenile plaice Pleuronectes platessa L. in Port Erin Bay, Irish Sea R.D.M. Nash*, A.J. Geffen Port Erin Marine Laboratory, School of Biological Sciences, University of Liverpool, Port Erin, Isle of Man IM9 6JA, UK Received 24 September 1999; accepted 15 April 2000
Abstract The interannual variability in settlement and mortality of juvenile plaice Pleuronectes platessa L. was investigated between 1992 and 1998 on Port Erin Bay, west side of the Isle of Man, Irish Sea. The dampening in¯uence of factors operating on the nursery grounds was especially obvious in 1992 and 1996. In these years extremely high numbers of individuals settled, yet the population sizes in July were similar to other years. Thus the nursery ground processes were likely to be density dependent. Shrimp and crab densities were low in Port Erin Bay and probably had little predatory impact on young plaice. Crustacean densities were not signi®cantly related to winter temperatures. In the Irish Sea, year-class strength is determined during the nursery ground phase, in contrast to the North Sea where determination of year-class strength occurs prior to the nursery ground phase. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Irish Sea; plaice; Pleuronectes platessa; juvenile; settlement; growth; density-dependent mortality; population
1. Introduction Variations in recruitment of the European plaice, Pleuronectes platessa L., have been attributed to different processes acting on early survival. The young stages of plaice successively occupy two different environments. As eggs and larvae, plaice occupy pelagic environments whereas juveniles occur in demersal habitats in the sandy beach areas that they use as nursery grounds. It has been suggested that interannual variability in plaice recruitment is generated in the pelagic phase (Zijlstra and Witte, 1985; Van der Veer, 1986) and the nursery ground phase dampens variability. The majority of the * Corresponding author. E-mail address: [email protected] (R.D.M. Nash).
research on juvenile plaice has concentrated on North Sea stocks and thus most ideas concerning juvenile plaice population dynamics have been tested in the North Sea. Fish populations in different areas can exhibit different strategies (Nash et al., 2000), and the mechanisms determining year-class strength in North Sea populations may not be the same as for other areas. The majority of research on juvenile plaice on nursery grounds comes from a limited number of locations, the largest of which is the Dutch Wadden Sea (e.g. Kuipers, 1977; Zijlstra et al., 1982; Van der Veer, 1986; Van der Veer and Bergman, 1986, 1987; Van der Veer et al., 1990, 1994, 1997). The processes which operate on Wadden Sea nursery grounds may be distinctly different from those operating on nursery grounds around the British Isles (Van der Veer et al.,
1385-1101/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 1385-110 1(00)00044-7
102
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
1990). The largest data sets from the British Isles come from Filey Bay on the northeast coast of England (Lockwood, 1974), the west coast of Scotland (Edwards and Steele, 1968; Gibson, 1973; Gibson et al., 1993, 1996; Gibson and Robb, 1996) and the Irish Sea (Bowers, 1963; Jones and Kain, 1964; Riley and Corlett, 1965; Colman, 1966; Alhossaini et al., 1989; Nash et al., 1992; Nash et al., 1994a,b; Dau, 1994; Ellis, 1994; Hyder and Nash, 1998; S. De la Rosa, unpublished MSc thesis, University of Geneva). In both the North and Irish Seas, plaice spawn in the early part of the year (January±April) (Simpson, 1959a,b) and newly metamorphosed individuals settle in April to July onto sandy bays which serve as nursery grounds (Riley and Corlett, 1965; Colman, 1966; Hyder and Nash, 1998). At metamorphosis, plaice move from a relatively dilute, three-dimensional environment to a relatively concentrated twodimensional environment. Since there is a marked increase in density after settlement there has been considerable effort spent on determining the in¯uence of density-dependence in mortality during the nursery ground phase (Lockwood, 1980; Zijlstra et al., 1982; Van der Veer, 1986; Iles and Beverton, 1991; Beverton and Iles, 1992a,b). Both shrimp (Crangon crangon) and crabs (Carcinus maenas) have been implicated as major predators of young plaice (Van der Veer and Bergman, 1987). Although observations consistent with density-dependent effects have been noted, actual causal relationships have yet to be demonstrated. Port Erin Bay is a small (450 m wide) sandy bay on the south-west corner of the Isle of Man in the Irish Sea. This small bay has been the subject of many studies, particularly of the juvenile plaice population (see above). Much of the work has considered juvenile plaice in isolation from the rest of the assemblages present in the bay. In this paper we examine variation in instantaneous mortality rate both within the nursery ground phase and between years. We critically evaluate the hypotheses concerning the regulatory in¯uence of shrimp and crab populations on plaice nursery ground survival. Lastly, we determine at what point in the life cycle of plaice year-class strength is determined.
2. Methods 2.1. Juvenile numbers in Port Erin Bay Population sizes of juvenile plaice in Port Erin Bay for 1991 were taken from Ellis (1994). From 1991± 1998 ®ve transects were sampled in Port Erin Bay with a 1.5 m beam trawl between April and July and then a 2 m beam trawl between July and November (Fig. 1). The 1.5 m beam trawl had a 5 mm stretch mesh throughout and the 2 m beam trawl had 9 mm stretched mesh throughout. The gear was changed to accommodate the seasonal changes in ®sh size to avoid net clogging from accumulating vegetation. Sampling intensity varied between the years, from fortnightly in 1991±1992 to weekly (when weather permitted) between 1993 and 1998. Each transect was sampled once on each date. Both beam trawls were ®tted with triple tickler chains. All captured ®sh, C. crangon and crab species were counted. Plaice (total length) and C. crangon (carapace length) were measured to the nearest 1 mm. The abundance of plaice was calculated, for each transect separately, by multiplying the estimated density by the area of the bay which was sampled and known to be occupied by O-group plaice (101 000 m 2) (see Fig. 1). The total population size was estimated from the mean of all ®ve transects with its associated standard deviation. Ef®ciencies of the nets were assumed to be 20% prior to June and 30% thereafter (Edwards and Steele, 1968). These are the only published ef®ciencies applicable to this site. C. crangon and crab densities during plaice settlement were estimated as the mean density between May and July each year. Sea-surface temperature data for Port Erin Bay were collected daily and supplied by T. Shammon (Port Erin Marine Laboratory, Isle of Man). 2.2. Mortality rates Daily instantaneous mortality rates were calculated from: M loge Nt =N0 =Dt where M is the instantaneous daily mortality rate, Nt is the number at time t, N0 is the number at time 0 and Dt is the time period in days.
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110 55
SCOTLAND
IRELAND
ISLE OF MAN
54
ENGLAND
IRISH SEA
53
103
on the beach, and these are the normally quoted nursery ground mortality rates for juvenile plaice (Beverton and Iles, 1992a,b). We calculated an early mortality rate (EMR), covering the early part of the season, to account for the mortality that occurs during the settlement period, when the ®sh are potentially more vulnerable. The EMRs were calculated from the difference between the maximum population size and the total number of newly settled individuals: EMR loge Nmax =Nns =Dt
WALES
-6
-5
-4
T5
-3
where Nmax is the maximum number of individuals, Nns the sum of newly settled individuals between May and July, de®ned as ®sh ,17 mm (Nash and Geffen, 1999), and Dt constitutes the number of days between the date of 50% settlement (based on the cumulative percentage of ®sh ,17 mm), and the date of maximum population size. 2.3. Recruitment index of plaice in the Irish Sea
LW
Port Erin Bay HW T1
200m
Fig. 1. Port Erin Bay, Isle of Man, Irish Sea. The ®ve transects (T1± T5) sampled between 1992 and 1998 are shown. LW low water, HW high water. Area of the bay used to calculate population size is bounded by the dotted line and the low water mark (LW).
Nursery ground mortality was calculated for the settlement period (approximately April±June) in addition to the standard mortality estimate. The standard mortality rates (SMR) were calculated from the slope of the decline in population size over time from the date of the maximum population size (usually July) through to either October or November each year. The SMR estimates only include mortality that occurs after the majority of the ®sh have settled
The relationship between the nursery ground plaice populations and estimates of recruitment was examined to evaluate the role of nursery ground processes in determining year-class strength in the Irish Sea. Recruitment was de®ned as the mean catch of 1year-old plaice in the Irish Sea from the RV Corystes (CEFAS, Lowestoft) beam trawl surveys. Data on catches of 1-year-old plaice were obtained from the CEFAS September beam trawl series (RV Corystes). These data are also reported to the ICES Northern Shelf Demersal Working Group (ICES, 1998). Using the age-1 catch data rather than Virtual Population Analysis output for 1-year-old plaice abundance avoids bias due to the analytical technique used to estimate population size. 3. Results 3.1. Juvenile numbers in Port Erin Bay The population size of juvenile plaice in Port Erin Bay ¯uctuated by one order of magnitude between 1991 and 1998 (Fig. 2). The lowest population size was recorded in 1993 (maximum of approximately 40 000 individuals or a density of approximately 0.4 m 22) and the largest in 1996 (maximum of
104
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
Fig. 2. Variation in population size of O-group plaice, Pleuronectes platessa, in Port Erin Bay, Isle of Man between January 1991 and December 1998. Error bars 1 standard deviation.
304 300 individuals or a density of approximately 3.0 m 22).
There was a signi®cant positive relationship between the numbers of plaice that settle (Nns) and the subsequent EMR (Fig. 3a).
3.2. Mortality rates
EMR 5:68 £ 1025 £ Nns £ 1023 1 8:4 £ 1024
3.2.1. Early mortality rates (EMR) The EMR varied considerably between 1991 and 1998. The lowest early instantaneous daily mortality rate occurred in 1998 and the highest in 1996 (Table 1).
F 1;7 151:034; P , 0:01: The two years with very large settlement (1992 and 1996) had the highest instantaneous mortality rates.
Table 1 Estimated instantaneous mortalities of juvenile plaice in Port Erin Bay, Isle of Man, Irish Sea. EMR is estimated from the total number of settling ®sh. SMR is estimated from the decrease in juvenile abundance from the time of maximum population size through to the autumn Year
Early mortality rate (EMR)
Standard mortality rate (SMR)
Instantaneous mortality (d 21)
Instantaneous mortality (d 21)
R2
P
0.0318 0.0770 0.0248 0.0309 0.0207 0.0862 0.0234 0.0172
0.0161 0.0132 0.0235 0.0123 0.0108 0.0160 0.0238 0.0290 0.0212 0.0100 0.0160
0.97 0.98 0.95 0.917 0.835 0.879 0.783 0.953 0.929 0.754 0.932
, 0.05 , 0.05 , 0.01 , 0.01 . 0.05 , 0.01 , 0.01 , 0.01 , 0.01 , 0.01 . 0.05
a
1963 1964 a 1965 a 1991 1992 1993 1994 1995 1996 1997 1998 a
Data from Iles and Beverton (1991).
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
a
0.10
105
1993±1995, 1997±1998 where:
Early mortality rate (EMR; d-1)
EMR 25:9 £ 1023 £ Feb: temp 1 6:84 £ 1022 0.08
1992
1996
0.06
0.04
1991 1997
0.02
1994 1993 1995
1998 0.00 0
400
800 1200 Settlement Index (Nns)
1600
0.10
Early mortality rate (EMR; d-1)
b 1996
0.08
1992
0.06
0.04 1994 1991
1997 1993
0.02
0.00 6 7 8 9 Average February water temperature (oC)
3.2.2. Standard nursery ground mortality rates (SMR) Standard daily instantaneous mortality rates in Port Erin Bay ranged from 0.010 d 21 for the 1997 yearclass to 0.029 d 21 for the 1995 year-class (Table 1). There was a similar range for data collected for the 1963-1965 year-classes in Port Erin Bay. Density-dependent mortality during the later part of the nursery ground phase was examined by regressing the maximum estimated population size (after settlement was completed) against the SMR (Fig. 4a). There was no relationship between the maximum population size and the subsequent nursery ground mortality F 1;7 1:938; P . 0:05: The SMR was independent F 1;10 0:325; P . 0:05 of mean temperatures between May and August (Fig. 4b). There was no trend in the relationship between the February water temperature and the SMR F 1;7 0:018; P . 0:05 (Fig. 4c). The inclusion of data generated by Riley and Corlett (1965) did not improve the relationship F 1;10 0:005; P . 0:05: 3.3. Predation by shrimp and crabs
1998 1995
5
F 1;5 96:10; P , 0:01:
10
Fig. 3. (a) Relationship between the EMR (instantaneous) and the numbers of juvenile O-group plaice that settle in Port Erin Bay. (b) Relationship between the EMR (instantaneous) of plaice in Port Erin Bay and the preceding February sea-surface water temperature (8C).
The relationship between EMR and February water temperature described two data groups; years with high mortality rates and years with low mortality rates. In general, EMR decreased with increasing February water temperature (Fig. 3b). There was a signi®cant inverse relationship between February water temperature and EMR for the years 1991,
Shrimp and crab densities in Port Erin Bay varied between years (shrimp CV: 29%; crab CV: 65%). Shrimps were approximately an order of magnitude more abundant than crabs. The variability in instantaneous mortality rates of plaice over these years (1995±1998) was 93% (CV) for EMR and 42% (CV) for SMR. There were no signi®cant relationships between the density of shrimps or crabs and either estimate of plaice instantaneous mortality rates. The density of C. crangon . 17 mm TL (i.e. those thought to be capable of predating young plaice, length frequencies of C. crangon can be found in Oh et al., 1999) was at most 0.1 m 22 (1995), a maximum of approximately 0.04 m 22 in 1996 and 1998 and very low (0.01 m 22) in 1997. These compare with maximum densities of settling plaice over the same time period of approximately 1.5 m 22 (1995), 3.5 m 22 in 1996 and 0.75 m 22 for 1997 and
106
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110 0.032
a
Standard mortality rate (SMR; d-1)
1995 0.028
0.024
1994 1996
0.020
1993
0.016
1998 1991 0.012
1992
1997
1998. The highest density of all size classes of C. crangon was approximately 1.8 m 22 in August 1995. Over the same time period, the highest density of crabs (Carcinus maenas, Liocarcinus puber, L. depurator, L. holsatus combined) was 0.05 m 22 in August 1996. There were no signi®cant relationships between shrimp or crab densities and the February water temperature F 1;3 1:59; 2.07; P . 0:05 for both) (Fig. 5(a) and (b)). There was, however, a signi®cant inverse relationship between mean shrimp and crab densities F 1;3 37:47; P , 0:05 (Fig. 5c).
0.008
0
100000
200000
300000
400000
Maximum population size
0.032
b
Standard mortality rate (SMR; d-1)
1995 0.028
1994
0.024
1965
1996 0.020
1998
1993
1963
0.016
1964 1991
0.012
1997
1992 0.008
10.8
11.2
11
12.0
12.4
12.8
13.2
Average May-August water temperature (oc)
0.032
c
1995 Standard mortality rate (SMR; d-1)
0.028
1994
0.024
1965
1996
3.4. Year-class strength There was no relationship between the numbers of plaice that settle in Port Erin Bay (at 1±4 months of age) and the numbers of 1-year-old plaice estimated from young ®sh surveys F 1;7 0:464; P . 0:05: Similarly there was no signi®cant relationship between the maximum population size (which can occur between May and August) seen in Port Erin Bay and the subsequent age-1 population F 1;7 2:820; P . 0:05: However, there was a signi®cant relationship between the population size in July (not always the maximum population size) in Port Erin Bay and the subsequent population size at age-1 F 1;6 10:186; P , 0:05 (Fig. 6a). There was a signi®cant relationship between the numbers of juveniles which settled and the maximum population size F 1;7 6:226; P , 0:05 (Fig. 6b) but not with the population size in July F 1;7 0:540; P . 0:05 (Fig. 6c). This suggests that the early part of the nursery ground phase is critical in determining yearclass strength. Further evidence for year-class strength being ®xed during the nursery ground phase comes from analysis of the interannual variability in population size at different stages. There was relatively little variation
0.020
1993
1963
1998
0.016
1964
1991 0.012
1992 1997
0.008
4
5
6
7
8
9
Average February water temperature (oC)
10
Fig. 4. (a) The relationship between SMR (instantaneous) and the maximum annual O-group plaice population size in Port Erin Bay. (b) Relationship between the SMR (instantaneous) of plaice in Port Erin Bay and the ambient sea-surface temperature between May and August in Port Erin Bay. (c) Relationship between the SMR (instantaneous) and the average February sea-surface temperature. Circles data collected in this study, closed circles signi®cant relationships to calculate mortality rates, open circles non-signi®cant relationships, crosses data from Iles and Beverton (1991).
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
a
0.40
Crangon density (ind m-2)
0.36
1998 0.32
1997
0.28
107
(CV 20%) in the estimated Spawning Stock Biomass (SSB) in the Irish Sea between 1990 and 1998. Over this time period, the numbers of ®sh settling in Port Erin Bay and the maximum population sizes were highly variable (CV 62 and 58%, respectively). However, the interannual variation (CV) in population size at age-1 was only 29%, close to the level observed for the adult population.
0.24
4. Discussion
1996
0.20
1995 0.16 6.8
7.2
7.6
8.0
8.4
8.8
9.2
February water temperature (oC) 0.016
b
1996 1995
Crab density (ind m-2)
0.012
0.008
1997 0.004
1998 0.000 6.8
7.2
7.6
8.0
8.4
8.8
9.2
February water temperature (oC) 0.016
c
1996 1995
Crab density (ind m-2)
0.012
0.008
1997 0.004
Interannual variability in population size is common in juvenile plaice populations (Bergman et al., 1988; Van der Veer et al., 1990; Modin and Pihl, 1994) and Port Erin Bay is no exception. The population sizes recorded between 1991 and 1998 are similar to those observed in the 1960s (Jones and Kain, 1964; Riley and Corlett, 1965). Maximum densities of Ogroup plaice in Port Erin Bay ranged from 0.4 m 22 in 1993 to 3.0 m 22 in 1996. The maximum densities and the range are comparable to the range and magnitude recorded for the Balgzand tidal ¯ats of the Dutch Wadden Sea (Bergman et al., 1988) suggesting that similar density-related processes may occur in both areas. The maximum densities recorded for both the Wadden Sea and Port Erin Bay are lower than some recorded for a small Swedish bay (Gullmar Bay, 1.3± 10.0 ind. m 22, Iles and Beverton, 1991; Modin and Pihl, 1994), comparable to Ardmucknish Bay (west coast of Scotland, 1.5±3.5 ind. m 22, Iles and Beverton, 1991) and higher than Filey Bay (east coast of England, 1.2 ind. m 22, Iles and Beverton, 1991). Densities are generally mean values integrated over the horizontal distance of a beach area and depth. Spatial differences in abundance (e.g. Lockwood, 1974; Riley et al., 1986) imply that the density experienced by individuals may be higher and lower than the reported mean values. The daily instantaneous mortality rates recorded for Port Erin Bay (0.011±0.029 d 21) are within the range reported in the literature for a number of other nursery grounds (0.007±0.052 d 21; Iles and Beverton, 1991).
1998 0.000 0.16
0.20
0.24 0.28 0.32 Crangon density (ind m-2)
0.36
0.40
Fig. 5. Relationships between the average February sea surface temperature in Port Erin Bay and the mean density of: (a) Crangon crangon; (b) crabs; and (c) the relationship between C. crangon and crab densities.
108
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110 14
a
Recruitment index (Irish Sea)
1995
12 1996
10 1997
1994
1992 1991
8 1993
6 0
40000
80000
120000
160000
Population size in July (n) 400000
Maximum population size (Nmax)
b
300000
1996
200000
1995 1994 100000
1997 1991 1998
1993
1992
0
0 160000
400 800 1200 Settlement index (Nns)
1600
c Population size in July (n)
1995 1996
120000
1994
80000
1997 1991
40000
1993
1992
1998 0 0
Apart from the two mortality estimates of Alhossaini et al. (1989) for Red Wharf Bay in 1986 and 1987 it appears that mortality on Irish Sea nursery grounds is within the range shown at Port Erin. Daily instantaneous mortality also varies between years. In Port Erin Bay, mortality rates may be more in¯uenced by density-dependent factors since elevated mortalities were observed at higher densities. There was only a weak relationship between juvenile plaice survival and winter temperatures in Port Erin Bay. In the Wadden Sea cold winter/spring conditions reduce the shrimp and crab populations and the predation on plaice is subsequently low (Van der Veer et al., 1990; Ansell and Gibson, 1993). In Port Erin Bay there was no relationship between the February temperature and instantaneous mortality, nor was there a relationship between temperature and the density of shrimp. Whether this is indicative of a different predation mechanism working in open `British bays' as compared with the Wadden Sea or some other factor is unknown. Shrimp and crab predation in Port Erin Bay have a much smaller effect on young plaice dynamics than may occur in other areas. C. crangon densities recorded between 1995 and 1998 were very low (maximum 1.8 m 22) when compared with Ardmucknish Bay in 1990 (maximum 6.6 m 22, Ansell et al., 1999) and the long-term average (22 years) for 15 stations in the Wadden Sea (approximately 60 m 22, Beukema, 1992). Similarly the maximum crab densities in Port Erin Bay (0.05 m 22) were considerably lower than in Ardmucknish Bay (0.88 m 22, Ansell et al., 1999). Oh et al. (2000) examined the stomachs of shrimp in Port Erin Bay for the period 1995±1998 and showed that approximately 18±20% of large shrimps had ®sh remains in their stomachs. The incidence of young plaice (10±15 mm) in shrimp stomachs is higher in the Wadden Sea (Van der Veer et al., 1994, 1997) and may be a consequence of higher shrimp densities there. Fish predators have some impact on juvenile plaice (Gibson and Robb, 1996). In Port Erin Bay (1995±
400 800 1200 Settlement index (Nns)
1600
Fig. 6. Relationships between: (a) the recruitment index for age-1 plaice in the Irish Sea and plaice population size in Port Erin Bay in July; (b) the maximum recorded population size in Port Erin Bay and the settlement index (Nns); and (c) the O-group plaice population size in Port Erin Bay in July and the settlement index (Nns).
R.D.M. Nash, A.J. Geffen / Journal of Sea Research 44 (2000) 101±110
1998) there were young plaice in the stomachs of whiting (Merlangius merlangus), cod (Gadus morhua), saithe (Pollachius virens), pollack (P. pollachius), poor cod (Trisopterus minutus), ®ve-bearded rockling (Ciliata mustela) and I 2 group plaice (P. platessa) (S. De la Rosa, unpublished MSc thesis, University of Geneva). The predators were primarily I 1 group ®sh. The impacts of this predation and potential predation by birds (see Van der Veer et al., 1997) have not, however, been quanti®ed. Thus the evidence available suggests that density-dependent mortality may be due to predation (perhaps by ®sh) and food availability may play an important role. The year-class strength in Irish Sea plaice is probably not ®xed until at least July and indications of year-class strengths are measurable on the nursery grounds. If the small Port Erin Bay population is indicative of nursery ground relationships for the Irish Sea as a whole, then interannual variability is generated primarily at stage-I egg production (Nash and Geffen, 1999) and it is not until into the nursery ground stage that the interannual variation decreases. The decrease in interannual variability indicated that year-class strength is ®xed during the nursery ground phase. In the North Sea year-class strength is measurable by the time of settlement, indicating that it is ®xed during the pelagic stages (Zijlstra and Witte, 1985; Van der Veer, 1986; Van der Veer et al., 1997). The dampening in¯uence of the nursery grounds, possibly due to density-dependent effects, was especially obvious in 1992 and 1996. In these years extremely high numbers of individuals settled, yet the population sizes in July were similar to other years. Port Erin Bay may be a good example of a nursery ground ®lter, where carrying capacity leads to density-dependent survival and the determination of year-class strength. In summary, the population dynamics of Port Erin Bay have many similarities to the dynamics of the Wadden Sea. In general, the mortality rates are higher than in the Wadden Sea but there is a certain degree of uniformity within the Irish Sea as a whole (especially for the smaller nursery grounds). The number of juvenile plaice which survive to the end of the ®rst year (and hence become the basis of year-class strength) is a function of the numbers present and the mortality rates. Therefore, an examination of mortality rates alone will not give insights into year-
109
class strength. Data from Port Erin Bay suggest that good year-classes can come from lower than expected mortality rates rather than low mortality rates. Acknowledgements We are grateful for the help many people have given during the sampling from 1990 to present, especially to Graham Hughes for the work undertaken over the whole time period reported here and Cheryl Corkill for the work undertaken between 1995 and 1998. We are grateful to T.R. Ellis (NERC CASE student Ð University of Liverpool and Dunstaffnage Marine Laboratory) and K. Dau (MSc student Ð University of Geneva, ®eld work with the University of Liverpool) for permission to use their Port Erin Bay plaice data. Data on settlement for 1991 were obtained during the tenure of an NERC grant (GR9/185) to RDMN and AJG and the data from 1995 to early 1997 were obtained during the tenure of an NERC grant (GST/02/902) to RDMN and from DML, Robin Gibson and Mike Burrows. References Alhossaini, M., Liu, Q., Pitcher, T.J., 1989. Otolith microstructure indicating growth and mortality among plaice, Pleuronectes platessa L., post-larval sub-cohorts. J. Fish Biol. 35 (Suppl. A), 81±90. Ansell, A.D., Gibson, R.N., 1993. The effect of sand and light on predation of juvenile plaice (Pleuronectes platessa) by ®shes and crustaceans. J. Fish Biol. 43, 837±845. Ansell, A.D., Comely, C.A., Robb, L., 1999. Distribution, movements and diet of macrocrustaceans on a Scottish sandy beach with particular reference to predation on juvenile ®shes. Mar. Ecol. Prog. Ser. 176, 115±130. Bergman, M.J.N., Van der Veer, H.W., Zijlstra, J.J., 1988. Plaice nurseries: effect on recruitment. J. Fish Biol. 33 (Suppl. A), 201±218. Beukema, J.J., 1992. Dynamics of juvenile shrimp Crangon crangon in a tidal-¯at nursery of the Wadden Sea after mild and cold winters. Mar. Ecol. Prog. Ser. 83, 157±165. Beverton, R.J.H., Iles, T.C., 1992a. Mortality rates of O-group plaice (Pleuronectes platessa L.), dab (Limanda limanda L.) and turbot (Scophthalmus maximus L.) in European waters. II. Comparison of mortality rates and construction of life table for O-group plaice. Neth. J. Sea Res. 29, 49±59. Beverton, R.J.H., Iles, T.C., 1992b. Mortality rates of O-group plaice (Pleuronectes platessa L.), dab (Limanda limanda L.) and turbot (Scophthalmus maximus L.) in European waters. III. Density-dependence of mortality rates of O-group plaice
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and some demographic implications. Neth. J. Sea Res. 29, 61± 79. Bowers, A.B., 1963. Growth of young plaice in Port Erin Bay. Ann. Rep. Biol. Port Erin. 75, 28±32. Colman, J.A., 1966. Studies on the biology of young plaice in Manx waters, PhD thesis, University of Liverpool. Dau, K., 1994. Population dynamics of O-group plaice, Pleuronectes platessa L., on a Manx nursery ground, including the study of sub-cohorts by use of otolith microstructure, MSc thesis, University of Geneva. Edwards, R.R.C., Steele, J.H., 1968. The ecology of O-group plaice and common dabs in Loch Ewe. I. Population and food. J. Exp. Mar. Biol. Ecol. 2, 215±238. Ellis, T.R., 1994. Production and mortality of early life stages of ¯at®shes, PhD thesis, University of Liverpool. Gibson, R.N., 1973. The intertidal movements and distribution of young ®sh on a sandy beach with special reference to the plaice (Pleuronectes platessa). J. Exp. Mar. Biol. Ecol. 12, 79±102. Gibson, R.N., Robb, L., 1996. Piscine predation on juvenile ®shes on a Scottish sandy beach. J. Fish Biol. 49, 120±138. Gibson, R.N., Ansell, A.D., Robb, L., 1993. Seasonal and annual variations in abundance and species composition of ®sh and macrocrustacean communities on a Scottish sandy beach. Mar. Ecol. Prog. Ser. 98, 89±105. Gibson, R.N., Robb, L., Burrows, M.T., Ansell, A.D., 1996. Tidal, diel and longer term changes in the distribution of ®shes on a Scottish sandy beach. Mar. Ecol. Prog. Ser. 130, 1±17. Hyder, K., Nash, R.D.M., 1998. Variations in settlement pattern of Irish Sea plaice (Pleuronectes platessa L.) as determined from a simulation model. J. Sea Res. 40, 59±71. ICES, 1998. Report of the Working Group on the Assessment of Northern Shelf Demersal Stocks, ICES CM 1999/ACFM:1 545pp. Iles, T.C., Beverton, R.J.H., 1991. Mortality rates of O-group plaice (Pleuronectes platessa L.), dab (Limanda limanda L.) and turbot (Scophthalmus maximus L.) in European waters. 1. Statistical analysis of the data and estimation of the parameters. Neth. J. Sea Res. 27, 217±235. Jones, N.S., Kain, J.M., 1964. The numbers of O-group plaice in Port Erin Bay in August 1963 observed by diving. Rep. Mar. Biol. Sta. Port Erin 76, 19±25. Kuipers, B.R., 1977. On the ecology of juvenile plaice on a tidal ¯at in the Wadden Sea. Neth. J. Sea Res. 11, 56±91. Lockwood, S.J., 1974. The settlement, distribution and movements of O-group plaice Pleuronectes platessa (L.) in Filey Bay, Yorkshire. J. Fish Biol. 6, 465±477. Lockwood, S.J., 1980. Density-dependent mortality in O-group plaice (Pleuronectes platessa L.) populations. J. Cons. 39, 148±153. Modin, J., Pihl, L., 1994. Differences in growth and mortality of juvenile plaice, Pleuronectes platessa L., following normal and extremely high settlement. Neth. J. Sea Res. 32, 331±341. Nash, R.D.M., Geffen, A.J., 1999. Variability in stage 1 plaice, Pleuronectes platessa, egg production on the west side of the Isle of Man, Irish Sea. Mar. Ecol. Prog. Ser. 189, 241±250. Nash, R.D.M., Geffen, A.J., Hughes, G., 1992. Winter growth of juvenile plaice in Port Erin Bay. J. Fish Biol. 41, 209±215. Nash, R.D.M., Santos, R.S., Geffen, A.J., Hughes, G., Ellis, T.,
1994a. Diel variability in catch rate of juvenile ¯at®sh on two small nursery grounds (Port Erin Bay, Isle of Man and Porto Pim, Faial, Azores). J. Fish Biol. 44, 35±45. Nash, R.D.M., Geffen, A.J., Hughes, G., 1994b. Individual growth of juvenile plaice on an Irish Sea nursery ground (Port Erin Bay, Isle of Man). Neth. J. Sea Res. 32, 369±378. Nash, R.D.M., Witthames, P.R., Pawson, M., Alesworth, E., 2000. Regional variability in the dynamics of reproduction and growth of Irish Sea plaice, Pleuronectes platessa L. J. Sea Res. 44, 55± 64 (this issue). Oh, C.W., Hartnoll, R.G., Nash, R.D.M., 1999. Population dynamics of the common shrimp (Crangon crangon L.) in Port Erin Bay, Isle of Man, Irish Sea. ICES. J. Mar. Sci. 56, 718±733. Oh, C.W., Hartnoll, R.G., Nash, R.D.M., 2000. Feeding ecology of the common shrimp (Crangon crangon L.) in Port Erin Bay, Isle of Man, Irish Sea. Mar. Ecol. Prog. Ser., in press. Riley, J.D., Corlett, J., 1965. The numbers of O-group plaice in Port Erin Bay, 1964±66. Rep. Mar. Biol. Sta. Port Erin 78, 51±56. Riley, J.D., Symonds, D.J., Woolner, L.E., 1986. Determination of the distribution of the planktonic and small demersal stages of ®sh in coastal waters of England and Wales and adjacent areas between 1970 and 1984. MAFF, Fish. Tech. Rep. 84, 1±23. Simpson, A.C., 1959a. The spawning of plaice (Pleuronectes platessa) in the Irish Sea. Fish. Invest. Lond. Ser. II 22 (8), 1±30. Simpson, A.C., 1959b. The spawning of plaice (Pleuronectes platessa) in the North Sea. Fish. Invest. Lond. Ser. II 22 (7), 1±111. Van der Veer, H.W., 1986. Immigration, settlement and densitydependent mortality of a larval and early post-larval plaice (Pleuronectes platessa L.) population in the western Wadden Sea. Mar. Ecol. Prog. Ser. 29, 223±236. Van der Veer, H.W., Bergman, M.J.N., 1986. Development of tidally related behaviour of a newly settled O-group plaice (Pleuronectes platessa) population in the western Wadden Sea. Mar. Ecol. Prog. Ser. 31, 121±129. Van der Veer, H.W., Bergman, M.J.N., 1987. Predation by crustaceans on a newly-settled O-group plaice (Pleuronectes platessa L.) population in the western Wadden Sea. Mar. Ecol. Prog. Ser. 35, 203±215. Van der Veer, H.W., Pihl, L., Bergman, M.J.N., 1990. Recruitment mechanisms in North Sea plaice, Pleuronectes platessa. Mar. Ecol. Prog. Ser. 64, 1±12. Van der Veer, H.W., Berghahn, R., Rijnsdorp, A.D., 1994. Impact of juvenile growth on recruitment in ¯at®sh. Neth. J. Sea Res. 32, 153±173. Van der Veer, H.W., Ellis, T., Miller, J.M., Pihl, L., Rijnsdorp, A.D., 1997. Size selective predation on juvenile North Sea ¯at®sh and possible implications for recruitment. In: Chambers, R.C., Trippel, E.A. (Eds.). Early Life History and Recruitment in Fish Populations. Chapman and Hall, London, pp. 279±303. Zijlstra, J.J., Witte, J.IJ., 1985. On the recruitment of O-group plaice in the North Sea. Neth. J. Zool. 35, 360±376. Zijlstra, J.J., Dapper, R., Witte, J.IJ., 1982. Settlement, growth and mortality of post-larval plaice (Pleuronectes platessa) in the western Wadden Sea. Neth. J. Sea Res. 15, 250±272.