(Gadus Morhua), Haddock (Melanogrammus Aeglefinus) and Whiting (Merlangius Merlangus) in the Northern North Sea

ICES CM 2011/ H:07 Not to be sited without prior reference to the authors.

Temporal settlement patterns and size at transition of the juvenile cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) in the northern North Sea.

Dorota K. Demain ¹*, Alejandro Gallego ¹, Colin Millar ¹, Imants G. Priede² and Emma G.

Jones 1,3

¹ Marine Scotland - Science, Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen,

AB11 9DB, United Kingdom. Tel. +44 (0) 1224 876544, Fax. +44 (0) 1224 295511

² University of Aberdeen, Oceanlab, Main Street, Newburgh, AB41 6AA, United Kingdom.

Tel. +44 (0) 1224 274408, Fax. +44 (0) 1224 274402

³ National Institute of Water & Atmospheric Research Ltd. (NIWA), PO Box 109-695, 269

Khyber Pass Road, Newmarket, Auckland 1149, New Zealand. Tel. +64 9 3752056, Fax. +64

9 375 2051

* E-mail: [email protected]

Keywords: settlement size, 0-group cod, 0-group haddock, 0-group whiting

1 ABSTRACT

Juvenile settlement, the transition from pelagic to demersal habitat, has been identified as an important milestone for cod, haddock and whiting but there is insufficient knowledge and data about this life stage. It is believed that the period of settlement has an impact on recruitment success, as important density-dependent processes may take place. Also, knowledge of settlement timing and duration is relevant to understanding population connectivity and thus to the development of successful conservation measures. To study the settlement ecology sampling was conducted over spring and summer of 2004, 2005 and 2006 at an inshore site off the east coast of Scotland. Over 4000 0-group cod, haddock and whiting were collected. The results showed that the transition from the pelagic to the demersal habitat was associated with clear and progressive changes in the prey composition of the juvenile fish. The size of fish that could be considered settled was estimated for cod at 4.9 (±0.3) cm, haddock at 7.8 (±0.4) cm and whiting at 8.5 (±0.6) cm. The results also suggested clear differences in the patterns of settlement between the different species. Juvenile cod were first to commence settlement in the study area, settled over one pulse lasting about a month and initially occupied shallower, inshore waters, while haddock settled in one short pulse lasting approximately two weeks and favoured deeper, further offshore locations. Whiting settled much later in the season and over protracted period of time and depth preferences varied over time and with increasing length.

INTRODUCTION

The timing of the arrival of juvenile fish in the nursery area is determined by several factors. The time and duration of spawning by adult fish (Brander 1994, 2005), water temperature dependent development of eggs and larvae (Iversen and Danielssen 1984, Pepin et al. 1997, Brander 2000, Fox et al. 2003, Geffen et al. 2006), hatch date (Lapolla and Buckley 2005), ocean currents and tides (Walford 1938, Kingsford et al. 1991, Shenker et al. 1993, Brander and Mohn 2004) and distance from the spawning ground (Jenkins et al. 1996) all influence when and in what numbers fish will become available for the transition to the demersal habitat. Also prey availability (Bailey and Houde 1989, Heath 1992, Brander et al. 2001,

2 Beaugrand et al. 2003, Platt et al. 2003), growth (Campana 1996, Gallego and Heath 1997, Shima and Findlay 2002), microhabitat selection (Lough et al. 1989, Tupper and Boutilier 1995a, Cote et al. 2001) and inter- and intraspecific competition for food and space (Grant and Brown 1998) can potentially influence the settlement and subsequent recruitment success.

A correlation between indices of larvae and juvenile fish abundance and subsequent recruitment was demonstrated by several studies (Helle et al. 2000, Begg and Marteinsdottir 2002, Jonasson et al. 2009). Traditionally the biggest importance to recruitment success was attached to the pelagic stages of gadoid early life history (Hjort 1914, Walford 1938, Cushing 1975, Bailey and Houde 1989, Sundby et al. 1989). However, the importance of subsequent settlement and the impact it has on the recruitment was recognised by Campana et al. (1989) Tupper and Boutilier (1995a, 1995b, 1997), Campana (1996) and Hüssy et al. (1997), who postulated that this stage of life is crucial for the future recruitment and that year-class strength is established around this time.

The aim of this study was to determine the timing of settlement; the length of time this process takes for the population and for individual fish and settlement sizes.

MATERIALS AND METHODS

1. Data collection 0-group cod, haddock and whiting were collected in Stonehaven Bay, an inshore North Sea nursery area, off the Scottish east coast. Samples were collected in the 2004 sampling season by pelagic and demersal trawling. The pelagic sampling started on 26 April, and from 12 May to 16 August it continued weekly. Demersal sampling commenced on 12 May and continued in 1-2 weekly intervals (mainly weekly) until 31 August. Sampling in 2005 consisted of four demersal and two pelagic sampling events, and in 2006 two demersal and two pelagic sampling events. Additionally, traps were deployed on five occasions in 2005 and three occasions in 2006. The dataset from the 2004 sampling season was the most comprehensive one, and was used to investigate changes in the patterns of density of 0-group cod, haddock and whiting in the pelagic and demersal zones over the settlement season. Data from the demersal trawls from the two following years (2005 and 2006) were only used to compare with the patterns observed in 2004. Details of gear used in field sampling are given in Demain et al. (2011).

3 Stonehaven Bay is covered with sand, with the exception of three natural rocky reefs perpendicular to the shore, that rise up to few meters above bottom and consist of rough- shaped rocks of various sizes, covered with soft corals and algae. The three reefs are located on the extension of geological fault running across Scotland known as Highland Boundary Fault (Craig 1991). Fish were sampled around these three sites characterised by increasing distance off shore. Depth ranged from 12 to 46 m overall. There were no overlapping depth ranges between sites in the case of the demersal samples collected in 2004. However, there was a depth gradient within each site due to bathymetry, tide and water level variability. Each site consisted of two different types of substrate: soft sand (“soft” substrate), where demersal trawls and traps were deployed and rocky reefs (“hard” substrate) where only traps were deployed. All fish caught were identified to species level and measured. Juvenile cod, haddock and whiting were preserved for further analyses and the remaining fish were discarded. From this point on, all references to juvenile fish refer to these three species, unless stated otherwise. Fish total length (TL) was measured to the nearest 0.1 mm using digital callipers and juveniles were subsequently grouped into 5 mm size classes.

2. Statistical methods For the purposes of investigating the temporal settlement patterns of 0-group fish density from pelagic and demersal catches were analysed. T-tests were carried out to test for differences in the density of 0-group cod, haddock and whiting between sampling seasons. For the purpose of statistical analysis of the temporal density patterns, demersal trawl data collected only at comparable dates in 2004 and 2005 were considered (i.e. data obtained between 2 June and 29 July in both years). The 2006 data were not used because sampling was carried out over a very short period of time and insufficient data were collected to allow comparison. The explanatory variables tested in density models were site, depth day-of-year and type of substrate.

The appropriate way of modeling population density based on count data which have been collected using a variable sampling effort is to use Generalized Mixed (Linear or Additive) Models using an appropriate distribution (Poisson with or without overdispersion and negative binomial) with a log link function and treating the log of the sampling effort as an offset variable. If we denote the underlying density at site and time by Dsite,time then we can calculate the density in terms of the observed counts N site,time and sampling effort E site,time

4 N site,time Dsite,time = (1) Esite,time

Solving the equation for count N site,time , log transformed:

log Nsite,time = log Dsite,time + log Esite,time (2)

The Generalized Mixed model fitted was:

log N ~ site + s(time) + log E (3)

Where density has been modelled as a function of site and a smooth function of time s, represented here by day-of year.

2.1. Temporal patterns of density and length distribution Models for pelagic and demersal data To model pelagic and demersal data, generalized linear and additive models for count data were used (Wood 2006). The pelagic data set consisted of single observations at each site over a series of weeks. Due to the scarcity of data, time was split into two periods and treated as a factor. The demersal trawl dataset consisted of weekly observations in which sampling took place twice at the same site in close temporal proximity. This is what is known as cluster sampling and leads to correlation within the pair of samples. The random component of this model allowed the constant and depth effect to differ for each site and sampling week.

Model selection The best predictive model was selected by forward and backward selection. Significance was assessed by the AIC (Akaike Information Criteria), as follows. The model chosen was the one with the fewest parameters within 2 units of the lowest observed AIC for the data set in question. This use of AIC for model selection has been published by Jones (1993). Significance of individual effects was tested using a Wald test, a multivariate version of z-test, which tests for a multivariate difference from zero when the effect has more than one parameter.

5 Length distribution of juveniles in trawl samples Samples collected by demersal trawls in 2004 were also used to investigate changes in the length distribution of 0-group fish over time (day-of-year) on the three sites. These data were analysed using linear regression and analysis of variance (ANOVA). The appropriate distribution for length data is Gaussian (Sokal and Rohlf 1995, Zuur et al. 2007). The assumptions of ANOVA were tested to ensure that independence, normality and homogeneity criteria were satisfied. The best predictive model was chosen by backward selection from the most complex model and forward selection from the model including only the global mean. The assessment of significance used the AIC (Akaike Information Criteria), with the selected model being the one with fewest parameters within 2 units of the lowest AIC (Jones 1993).

2.2. Dietary analysis In total, 2391 stomachs were analysed, of which 1866 were sampled in 2004 and 525 in 2005 (Table 1).

Prey types Prey items were identified to the lowest possible taxonomic level using the relevant sources (Naylor 1972, Russell 1976, Smaldon 1979, Lowry and Springthrope 2001, Meland 2002, Pan 2004). However, well-digested prey items were, in general, difficult to identify and required grouping of lower categories into higher taxonomic levels. The resulting taxonomic resolution was considered adequate to provide a general description of the diet composition. This approach was consistent across samples, with distinct prey types accounted for wherever possible. Highly digested jelly-like matter was recorded as “Unidentified”, weighed and counted as a single prey item. Empty stomachs formed an additional category. Two methods were used to provide a quantitative description of the diet: (i) the weight of each prey category in a stomach was measured on a microgram balance (to the nearest 0.1 mg) and expressed as a percentage of the total weight of stomach contents (gravimetric approach) and (ii) the number of each prey category in a stomach was recorded and expressed as a percentage of the total number of items found in a stomach (numerical approach). In total more stomachs were analysed by the numerical rather than the gravimetric approach (2346 and 1850, respectively). This was due to the difficulties with obtaining weight readings for fish in the small size classes (1 to 3 cm), due to the small size of prey items. Also, most of the fish from the small size classes originated from the pelagic samples, which were preserved in

6 alcohol. Wet weight measurements of the small prey items in particular were not carried out due to the high likelihood of measurement error. Empty stomachs were not included in the dietary analysis. The numerical method was used in all the cases and the gravimetric method was also applied where weight measurements of prey items could be obtained. Comprehensive analysis of the diet was presented in Demain et al. (2011).

Pelagic versus benthic prey composition In order to determine the length at settlement of cod, haddock and whiting, changes in the relative abundance of pelagic and benthic prey with increasing size were examined. The relative importance of the pelagic and benthic prey categories was analysed in 0.5 cm size classes for juvenile fish sampled in all years (2004 and 2005) and by all gear types (pelagic and demersal trawls and traps). Prey types were classed as pelagic or benthic depending on the habitat of the prey item, at the time when samples were collected, based on information from the literature. Prey items of uncertain origin and those impossible to categorise (unidentified) were grouped into the category “Other”. Two indices were used to measure changes in relative importance of the pelagic and benthic prey: (i) the average weight percentage of each prey type (pelagic, benthic or other) was calculated as:

ny W % xj W % xy = ∑ , (4) j=1 n y

Wxj where W% xj is weight percentage of prey type x consumed by fish j: W % xj = *100 % W j (4.6) and Wxj is the weight of prey type x (pelagic, benthic or other) in fish j, Wj is the total prey weight in fish j and ny is the total number of fish analysed in size class y.

And (ii) the average number percentage of each prey type (pelagic, benthic or other) was calculated as:

ny N% xj N % xy = ∑ , (5) j=1 n y where N% xj is number percentage of prey type x consumed by fish j:

N xj N% xj = *100 % (6) N j

7 and Nxj is the number of prey type x (pelagic, benthic or other) in fish j, Nj is the total of the prey numbers in fish j and ny is the total number of fish analysed in size class y.

The settlement threshold was set at fish length where more than 50 % of benthic prey, by weight or number, were consumed (Bowman 1981). A binomial logistic regression was fitted to the dietary data with fish length as a covariate. Logistic regression fits a logistic S-shaped curve to the proportion of prey consumed. The following model was fitted to the proportion “P” of prey consumed:

 P  log  = a + b *length (7) 1− P  where a is an intercept and b is a slope associated with length of fish. From the model, (l50), the length at which the proportion of benthic prey consumed equals 50 % (P = 0.5) was calculated. The confidence intervals were estimated from the model and calculated as ±2SE (standard error). A similar process was followed using the proportion of the pelagic prey. The settlement interval is presented as the size range between l50 pelagic and l50 benthic.

RESULTS

1. Temporal patterns of density and length distribution Over the period 2004-2006 716 cod (15-160 mm), 1099 haddock (10-155 mm) and 2756 whiting (10-170 mm) were sampled. See Table 2 for the summary of numbers of 0-group fish caught by all three types of gear. Models fitted to pelagic trawl abundance data are presented in Table 3 and to demersal trawl abundance data in Tables 4-6, respectively. Trends in the length distribution were also analysed from the pelagic and demersal catches collected in the 2004 sampling season. The weekly sampling carried out that year provided a comprehensive dataset on size frequency of juveniles. The changes in the length distribution through time were different for all three species. The choice of models best describing the length distribution of juvenile fish in demersal trawls in 2004 is presented in Tables 7-9, respectively.

8 Cod With the exception of 2005 demersal catches cod were the least abundant gadoids of the three species of interest. Cod were first caught in the pelagic samples on 17 May 2004 (day 138) and last on 14 June 2004 (day 166). The smallest cod was 15 mm long. The length of cod juveniles increased through the season. The maximum size of cod juveniles in the pelagic zone was 40 mm and it was recorded for the first time on 24 May (day 145; Fig. 4). Due to the scarcity of data, observations were split into two periods (period 1 – before 15 June, period 2 – after 15 June; day 167), where 15 June was the mid-point of the sampling season. On the basis of the mixed model analysis it was concluded that there was a significant time effect on density of pelagic cod (Wald-test, p<0.05; Table 3). Density of cod in the pelagic zone was significantly higher (t- test, p<0.05) until mid-June (in the first period) than thereafter (in the second period). Their occurrence in the pelagic zone in 2004-2006 was consistent with the patterns observed in 2004 (Fig. 1 a). At the time when sampling in 2005 and 2006 took place, no fish were expected to be present in the pelagic zone. This was generally the case, with the exception of 27 July 2005, when two 40 mm cod juveniles were caught.

Cod were found in the demersal catches from the beginning of sampling (12 May 2004; day 133), a week earlier than in pelagic samples. The density of cod changed over time (Wald- test, p<0.05) on all three sites (Wald-test, p<0.05) after the initial settlement of juveniles (Fig. 1 b).

In 2004 cod were the smallest gadoids recorded on the seabed, at a length of 20 mm (Fig. 5 A). The occurrence of cod in the 20-30 mm size range was quite common in the demersal zone through May and until mid-June. In 2004, in terms of maximum length, cod were the smallest of the three species. From mid-June an increase in the minimum and median size was recorded. This coincided with the disappearance of cod from the pelagic catches. Cod reached a maximum size of 100 mm in June, 125 mm in July and 135 mm in August (all seasons and all types of sampling gear considered). The occurrence of demersal 0-group cod in 2005 and 2006 was consistent with the patterns observed in 2004, although in 2005 a significantly higher abundance of juvenile cod was recorded (t-test; p<0.05).

The length distribution data from 2005 and 2006 obtained by demersal trawling and traps provide additional information to complement the 2004 dataset.

9 Data from 2005 were obtained by trawls and traps, and for 2006 mainly from traps, as only one cod was caught in the demersal trawls (27 July; day 208). The juvenile cod length data for 2005 and 2006, from mid-June until the end of sampling, show a pattern of gradual increase in minimum and median length (Fig. 5 B, C and D).

Combined data from the demersal trawls and trap catches for all three sampling seasons help to better visualise the general trends in cod size distribution over the settling season (Fig. 6). The statistical analysis of the combined data shows that the juvenile size was positively correlated with day-of-year (ANOVA, p<0.0001). However it also indicates that there was a gear selectivity effect (traps caught bigger fish than trawls), and a year effect (fish caught in 2005 and 2006 were bigger than in 2004; ANOVA, p<0.0001). Therefore the combination of all data sources can only be used for illustrative purposes.

Most cod settled at the shallowest depth, closest to the shore (Site 1) and at the intermediate depth and intermediate distance offshore (Site 2) at the beginning of the sampling season, and subsequently numbers of cod declined on both sites. Also, at Site 3, the deepest and farthest offshore, the density of fish declined over time. The density of cod on this site was significantly lower (t-test, p<0.05) than on the two other sites. There was no increase in the numbers of fish in deeper waters (Sites 2 or 3) coinciding with a decline of fish in the shallowest depth (Site 1) which implies that cod moved into deeper waters, farther offshore, as the season progressed but this observation in itself does not rule out the possibility of offshore movement.

There were significant (F-test, p<0.001) changes in fish length over time. The length increased over time on all three sites. There were, however, significant differences (F-test, p<0.001) between sites. Cod had a larger mean length on deeper sites (Site 2; t-test, p<0.001 and Site 3; t-test, p<0.05) than on Site 1. The length distribution, combined with abundance patterns in pelagic and demersal zones, indicate that cod probably settled in a single pulse lasting from mid-May to mid-June in 2004.

Haddock Of the three species of interest, haddock were the most abundant gadoid juveniles in the 2004 pelagic catch. The smallest juveniles were 10 mm long, and they were sampled in the study area for the first time on 26 April (day 117; the earliest of all three gadoid species) at the

10 beginning of the sampling period. Numbers and sizes of 0-group haddock subsequently increased, reaching a peak of abundance and maximum size of 45 mm reached in pelagic zone on 17 May (day 138). No larger haddock were captured in pelagic catches in this study. Subsequently the abundance of haddock declined and they became absent from pelagic catches by 7 June (day 159; Fig. 2 a). Similarly to the case of cod, data were divided into two time periods (before and after 15 June). Time was the only significant effect (Wald-test, p<0.05; Table 3), with juveniles being significantly more abundant in the first period (t-test, p<0.05). From the patterns of abundance of pelagic haddock in 2004 it was expected to find no haddock juveniles in the pelagic zone at the time sampling was carried out in 2005 and 2006. Consistently with expectations no fish were found in the samples.

Haddock were present in the demersal catches from mid-May 2004, at the beginning of demersal sampling. The increase in the numbers of juvenile haddock in the demersal catches coincided with their decrease in the pelagic catches. The density of haddock changed over time (Wald-test, p<0.05) and there were significant differences between the sites (Wald-test, p<0.05; Fig. 2 b). Time was fitted as a smooth function, as the density changes over time were continuous but non-linear. The plot of the predicted changes in the density of 0-group haddock with time at different depths (sites) show that there was a sudden increase in the abundance of juveniles, particularly in deeper waters (Sites 2 and 3), from the beginning of June until the peak of abundance at the beginning of July. It was followed by a decline in the numbers of haddock to an almost constant level from mid-July. Intermediate depths (Site 2) and, to a lesser degree, deeper waters (Site 3) seemed to be particularly favoured. The density of haddock on Site 1 was significantly lower (t-test, p<0.05) than on the other sites. At the beginning of the sampling season fish were present on all three sites, but after 6 July (day 188) haddock were caught only on Sites 2 and 3.

From the beginning of sampling until 25 May (day 146) the presence of the smallest juveniles (between 30 and 45 mm) in the demersal zone was observed. From 2 June (day 154) there was an increase in minimum size of sampled haddock, which continued until the end of sampling in 2004. This, combined with the last pelagic haddock being sampled on 1 June (day 153), suggests that they settled in one pulse that lasted until the end of May - beginning of June.

11 There were significant differences in lengths between different sites (F-test, p<0.001). There was also a significant interaction term between site and time (F-test, p<0.001), indicating that length distributions differed between sites with time. From the beginning of the sampling season until 6 July (day 188), there was a similar length change pattern on all sites. After this date fish of increasing length were sampled on Sites 2 and 3 but were not present on Site 1. Fish on Site 1 were significantly smaller than on other two sites (t-test, p<0.001). Fish with the largest mean length were found initially on intermediate Site 2 and then, towards the end of sampling season, on Site 3, the deepest and farthest away from shore. The pattern of haddock size distribution suggests that fish moved towards deeper waters with increasing size.

Results in 2005 and 2006 were consistent with the patterns of abundance of demersal haddock in 2004. In the 2005 sampling season, a significantly (t-test; p<0.05) higher abundance of juvenile haddock was observed than in 2004. Also the length distribution of haddock juveniles in 2005 and 2006 was similar to the patterns observed in 2004 (Fig. 8 A-C).

Whiting Whiting were the most abundant juvenile gadoid (of the three species of interest) in the study area in all three years of study. Whiting were first caught in the pelagic zone on 12 May 2004 (day 133). Their peak of abundance was observed on 17 May (day 138). The last pelagic whiting were caught on 27 July 2004 (day 208; Fig. 3 a). Pelagic juvenile whiting ranged between 10-60 mm in size in 2004. Between the beginning of sampling and 14 June (day 166) a gradual increase in the maximum size of juveniles was recorded. Throughout sampling season small whiting were reappearing in the pelagic samples. The abundance data were divided into two periods – before and after 15 June (day 167). Preliminary models indicated that there might be time and site effects on the juvenile density. Small differences between AIC values in all tested models led to further exploratory analysis. A second exercise of model selection was carried out after removing the most extreme observation (whiting* in Table 3). This analysis led to the conclusion that there were no time or site effects and that density was constant throughout the area during sampling period. In the 2005 sampling season occurrence of juvenile whiting in the pelagic zone was consistent with the pattern observed in 2004.

In the 2004 demersal catches whiting were found from the beginning of June, three weeks after they were first detected in the pelagic zone. The size of juveniles in demersal samples

12 ranged between 30 and 145 mm. The density and maximum size of 0-group whiting increased throughout the sampling season (Fig. 3 b). The patterns of density over time were different among sites (indicated by the interaction between site and time, with a smooth function, in the model; Wald-test, p<0.05) and showed a rapid increase in number of settling juveniles from 8 June (day 160). Between 8 June and 27 June (day 181), fish were present exclusively on Site 1. Only from this date there were whiting caught on all sites. The density of whiting on Site 1 from 15 July (day 197) reached a plateau at a level much lower than on the two other sites. On Site 2, after 6 July (day 188) numbers of fish increased rapidly, than decreased slightly and levelled towards the end of the season at lower densities than on Site 3. The increase in density of whiting on Site 3 was delayed relative to Site 2. However, numbers of fish increased until the end of the season to higher levels than on Site 2. Juvenile whiting on Sites 2 and 3 were significantly (t-test, p<0.05) more abundant than on Site 1. Also in the case of whiting there were significant differences in fish length between sites (F- test, p<0.001) and a significant interaction term between site and time (F-test, p<0.001), indicating that length distributions differed between sites with time. Before 27 June (day 181) small whiting were caught only on Site 1. Whiting caught on the deeper Sites 2 and 3 were over all significantly smaller than on Site 1 (t-test, p<0.05 and p<0.001, respectively). The size distribution of whiting, particularly the continuous presence of small juveniles in the study area until the end of July, combined with abundance patterns in the pelagic and demersal zones, indicate a protracted population settlement pattern lasting from the beginning of June until the beginning of August. The abundance and length distribution patterns of juvenile whiting in the demersal catches in 2005 and 2006 were consistent with the patterns observed in 2004. In 2005 a significantly (t- test; p<0.05) higher abundance of juvenile whiting than in 2004 was observed.

2. Relative importance of pelagic versus benthic prey Cod Cod smaller then 3.0 cm preyed exclusively on pelagic food items, which consisted entirely of copepods, mainly T. longicornis and C. finmarchicus. From a size of 3.0 cm juvenile cod began to feed on benthic prey items, such as C. crangon, megalopa larvae of crabs, fish (Ammodytes spp.) and euphausiids. However, pelagic food still constituted over 75 % of the diet weight (84 % of prey numbers).

13 Between 3.0 and 4.5 cm the importance of pelagic food decreased relative to that of benthic and by 5.0 cm over 60 % of consumed prey weight consisted of benthic prey. At this size interval the range of benthic prey consumed expanded and included prawns and shrimps, molluscs, cumaceans, crab larvae (megalopa), amphipods (mainly caprellids and gammarids) and plaice.

By 7.0 cm size class the average pelagic food fraction by weight dropped below 1 % and benthic prey remained > 80 % of prey weight (with four exceptions at 8.0, 9.5, 11.5 and 13.5 cm). At a length of 7.0 cm the average abundance of pelagic prey by numbers dropped to < 10 % (with one exception at 11.0 cm size class, when pelagic prey constituted 16.7 % of prey numbers). Benthic prey numbers were maintained at near 70 % or more, with two exceptions (at 11.5 and 13.5 cm). The exceptions were observed only in size classes with small numbers of stomachs analysed, but even then the decrease in content of benthic components coincided with increases in unidentified highly digested matter, not pelagic prey.

Generally, cod over 8.5 cm did not feed on pelagic prey. The two exceptions were found in stomachs analysed by the numerical approach, at size classes 11.0 and 12.5 cm, where single pelagic prey items were present.

The changes in relative weight and numbers of pelagic and benthic prey indicate that cod started feeding on benthic prey at 3.0 cm and by > 8.5 cm the diet was exclusively benthic. Settlement was a gradual process that took place in case of cod in the size interval 3.4 (±0.2) – 4.9 (±0.3) cm and by 4.9 (±0.3) cm (Fig. 11), as calculated from the logistic regression model, were considered settled (> 50 % of benthic dietary components by weight or number (Bowman, 1981)).

Haddock Juvenile haddock in size classes < 2.0 cm fed exclusively on pelagic prey (100 % of prey by weight and numbers), mainly Limacina spp., small copepods, T. longicornis, C. finmarchicus and ostracods.

Between 2.0 and 3.0 cm size interval the range of crustaceans consumed increased and included amphipods (hyperids), euphausiids (larvae calyptopis and furcila), eggs of invertebrates, zoea larvae of crabs, increasing numbers of copepods, particularly C.

14 finmarchicus, T. longicornis and Pseudocalanus spp. and occasional insects. Also at this time prey items classed as benthic were encountered in stomachs for the first time. In all instances in this size range, the benthic prey components were made up by Ammodytes spp. (in size class 3.0 cm they constituted over 60 % of consumed weight) and were found among juvenile haddock sampled by pelagic tows on 24 May 2004 (one 2 cm fish was caught on 17 May 2004, also in a pelagic tow).

From the 3.5 cm size class megalopa larvae of crabs, polychaetes, adult euphausiids and benthic fish (Pleuronectiformes) started to be consumed.

In the size range 5.0-5.5 cm there was an abrupt drop in the proportion of pelagic prey consumed. The average pelagic prey by weight dropped from 29.6 % at 5.0 cm size class to 1.9 % at 5.5 cm, and was maintained < 3 % in the following size classes. The average pelagic prey numbers fell from 50.1 % at 4.5 cm size class initially to 25 % at 5.5 cm, to 7.7 % by 8.0 cm and from 10.0 cm size class fluctuated between 0 and 3 %. Despite the fall in consumption of pelagic prey, an increase in proportion of benthic prey was not clearly apparent, mainly due to the increase of unidentified stomach content. Particularly in the 5.5-8.5 cm size classes highly digested jelly-like matter constituted between 50 and 85 % of prey weight. The majority of haddock with highly digested stomach content were sampled on 15 June 2004, 15 July 2004 and 23 June 2005.

The benthic components constituted over 50 % of the diet from 7.8 (±0.4) cm (by numbers; Fig. 12) as calculated from the logistic regression model when haddock were considered settled. The proportion of pelagic food in stomachs of haddock decreased below 50 % at 2.9 (±0.6) cm (by weight). In the case of haddock settlement was therefore also a gradual process that took place in the 2.9 (±0.6) – 7.8 (±0.4) cm size interval.

Whiting Juvenile whiting up to 2.5 cm fed exclusively on a pelagic diet, consisting of T. longicornis and small copepods (mainly C1-C4 stages of C. finmarchicus). From 2.0 cm also adult stages of C. finmarchicus and Limacina spp. were also present in the stomachs. Whiting as small as 2.5 cm also consumed Ammodytes spp. and occasional benthic prey items (polychaetes).

15 From 3.5 cm amphipods were also included in the diet and from 4.5 cm crab megalopa. Small copepods and T. longicornis, along with larger species like C. finmarchicus and C. armata, were the most important items in the diet of smaller whiting (up to 4.5 cm) but they were still present in significant numbers in larger size classes.

Food from a pelagic source constituted over 50 % of the diet up to a fish length of 3.0 cm by weight and 4.0 cm by number. There was one exception in stomachs analysed by gravimetric approach at the 3.0 cm size class, where benthic prey constituted 83.3 % by weight. Such a high proportion of benthic prey in this size category was mainly due to Ammodytes spp., that were generally classified as benthic. All 2.5-3.0 cm juvenile whiting that preyed on Ammodytes spp. came from a single pelagic tow carried out on 24 May 2004.

As the size of fish increased, so did the variety of consumed prey. Larger, heavier prey items like megalopa larvae of crabs, juvenile crabs, prawns and shrimps, amphipods, cumaceans and fish made up most of the prey weight.

From the 3.5 cm size class upwards the consumption of benthic prey gradually increased to exceed the 50 % threshold at 8.5 (±0.6) cm (by weight) as calculated from the logistic regression model. Benthic prey constituted the majority of food of 0-group whiting, by weight and number, from these size classes onwards. An exception was noted at size class 15.0 cm, with 3 fish analysed only numerically, where benthic food content fell to 19.4 %. However, even in this case, the decrease in the proportion of benthic prey found in the stomachs, coincided with an increase in unidentified matter, not an increase of pelagic prey.

Pelagic prey consumption by whiting dropped below the 50 % level at 2.9 (±0.6) cm (by weight). Settlement of whiting was a gradual process that took place in the 2.9 (±0.6) – 8.5 (±0.6) cm size interval, and by 8.5 (±0.6) cm (Fig. 13) whiting were considered settled according to the criterion of Bowman (1981).

Pelagic prey numbers constituted on average over 20 % of juvenile whiting prey up to the 11.5 cm size class and even in larger size classes they accounted for a significant amount of prey (only twice below 9 % at 13.0 and 16.0 cm). In stomachs analysed gravimetrically the average proportion of pelagic prey by weight fell below 10 % by 6.0 cm (with an exception at 11.0 cm), and below 1 % by 13.0 cm. This indicates that pelagic prey were numerous

16 contributors to the juvenile whiting diet through the entire size spectrum. No stomachs were analysed gravimetrically in the 1-2 cm and 15-17 cm size classes.

DISCUSSION

The spawning period of cod falls between January and April (Hislop 1984, Coull et al. 1998), of haddock between February/March and May (Hislop 1984, Coull et al. 1998), and of whiting between March and June/July (Hislop 1984) or February and June (Coull et al. 1998). The spawning time and duration are influenced by maternal characteristics like size and age (Wright and Trippel 2009). Information about the reported spawning time and duration, length of the pelagic phase, which lasts five months (Miller et al. 1963) and the time and length of settlement reported previously in the literature determined the initial choice of a sampling period spanning between the end of April and the beginning of September. The most intensive sampling took place in 2004. It provided weekly data on changes in the abundance and size frequency in the pelagic and demersal zone at the time of settlement for 0-group cod, haddock and whiting. The limited sampling that was carried out in the 2005 and 2006 sampling seasons provided the basis for comparison of annual abundance fluctuations at the time of settlement, and suggested the 2005 sampling season as a year of higher abundance in all three species.

Similarly to the case of reaction norms for maturation (Heino et al. 2002), settlement does not occur at a fixed length but happens gradually and can influenced by other factors such as resource availability and body reserve. In this study probabilistic estimates of size at settlement was undertaken using a logistic regression technique. According to the criterion of Bowman (1981) fish can be regarded as settled when more than 50 % of prey comes from a benthic source. The gradual decrease of food from pelagic source with simultaneous increase of benthic food content in stomachs indicates that juveniles prior to permanent switch of the habitat carried out exploratory migrations towards sea bed. For that reason the interval between pelagic prey decreasing below the 50 % threshold and benthic prey increasing above the 50 % threshold was regarded as a settlement period, when adaptation to the benthic mode of feeding takes place.

The results of this dietary analysis indicate that the settlement and following shift from pelagic to benthic type of prey was a gradual process, that occurred at the size range of 3.4 (±0.2) – 4.9 (±0.3) cm in cod. The size of fish that could be considered settled according to

17 the criterion of Bowman (1981) was estimated for cod at 4.9 (±0.3) cm, at smaller size than previously reported (Daan 1973 (5-9 cm), Bowman 1981 (> 9 cm), Hüssy et al. 1997 (5-7 cm) and Lomond 1998 et al. (6-10 cm)).

Juvenile cod were sampled in the pelagic zone between 17 May and 14 June 2004. However, in the demersal catches juveniles were found earlier (12 May), suggesting that there must have been pelagic cod present before that date. Very small numbers of 0-group cod caught in the pelagic zone and the absence of juveniles in catches before 17 May were most likely a result of the dispersion of juveniles and their low abundance. The peak of abundance of pelagic cod was in mid-May, followed by their final disappearance from the pelagic zone by mid-June.

Cod were found in the demersal catches from the beginning to the end of sampling in 2004 and there was a declining pattern of abundance over time. The drop in abundance may be due to a potential switch to rocky reef habitat by juvenile cod and/or mortality of young settlers. The preference for the structured habitat, that provides cover from predation, was previously reported by Grant and Brown (1998), Lindholm et al. (1999) and Laurel et al. (2003), among others. To determine habitat preference of cod in this study, trap sampling was planned. Unfortunately, traps designed and deployed in the 2004 sampling season were unsuccessful at sampling any juveniles.

The abundance of cod in the study area in the 2005 sampling season was significantly higher than in 2004. These findings must be treated with caution, as the comparison of the data from two years was based on the limited numbers of samples taken in 2005. These findings were, however, consistent with the data from the International Bottom-Trawl Survey (IBTS), the Scottish Ground Fish Survey (SCOGFS) and the English Ground Fish Survey (ENGGFS) (ICES 2007a,b). They all indicate that from the estimates of the 0-group cod recorded in 2005 and 1-group cod recorded in 2006, the 2005 year class was higher in abundance in the North Sea, particularly in the central and northern part, than recent low levels. IBTS data showed in 2005 the highest numbers of 0-group cod in quarter 3 (July-September) since 1998, and in 2006 the highest numbers of 1-group cod in quarter 1 (January-March) since 2002. SCOGFS and ENGGFS data showed in 2006 the highest numbers of 1-group cod in quarter 3 (July- September) since 2000 and 1998 respectively.

18 The length distribution of 0-group cod in the 2004 sampling season confirmed the general pattern derived from the abundance data. There was an initial inflow of small juveniles in the pelagic zone, with the largest cod recorded being 40 mm long. Specimens as small as 20-25 mm were regularly caught in the demersal zone up to mid-June. There was an increase in the size of cod with time in trawl samples, and that the largest cod occurred on Site 2, at intermediate depths. A further increase in size at the deeper Site 3 was not observed but, similarly to above, this could be due to the termination of sampling before the change took place, especially since the increasing affinity of cod for deeper waters with age is well documented (Riley and Parnell 1984, Tremblay and Sinclair 1985, Keats et al. 1987, Sinclair 1992, Hessen 1993, Linehan et al. 2001). This evidence suggests that juveniles settle initially close to the shore, in shallow waters, and move deeper as size increases. This results in at least a partial size segregation, which can reduce cannibalism and increase the chances of survival of 0-group cod (Riley and Parnell 1984). The size distribution pattern suggests that there may have been a single pulse of settlement, lasting one month, between mid-May and mid-June in the study area in 2004. This was consistent with the abundance data, where the presence of pelagic juveniles was recorded until mid-June. The absence of small, new arriving fish, indicates that after that period upward trend in length distribution was likely due to fish growth. Demersal catches of cod between the beginning of July and the end of August were very low. Therefore it is difficult to be certain that there were no more small juveniles settling. However, the absence of juvenile cod from the pelagic catches after mid-June supports the theory that the transition of cod to the seabed was completed by this time. The maximum size of pelagic juveniles, combined with the most common size of small cod in the demersal zone, leads to the conclusion that cod settlement occurred between mid-May and mid-June.

The settlement of haddock and following shift from pelagic to benthic type of prey was a gradual process also for haddock, that occurred at the size range of 2.9 (±0.6) – 7.8 (±0.4) cm, as indicated by the dietary analysis. The size at which haddock could be considered settled according to the criterion of Bowman (1981) was estimated at 7.8 (±0.4) cm.

In this study haddock was the first species sampled in the study area. Haddock were found from the beginning of sampling on 26 April. The peak of abundance in the pelagic zone was noted on 17 May, followed by their final disappearance from the pelagic zone by the first week of June. This short period of pelagic presence suggests that haddock settled in one pulse.

19 Coinciding with the decline in the numbers of haddock in the pelagic zone, there was an increase of these juveniles observed in the demersal zone. This reverse pattern of abundance between pelagic and demersal zone, with the first juveniles being sampled on the bottom in the first half of May and last pelagic juveniles being caught at the beginning of June suggests that the settlement of haddock is a process taking between one and two weeks and is completed by the beginning of June on the east coast of Scotland. It is not possible to be absolutely certain that there were no haddock present on the seabed earlier than 12 May 2004. Earlier presence of juveniles would imply that settlement starts earlier. However there was only a single 10 mm specimen sampled in pelagic zone on 26 April and two haddock sampled in the demersal zone on 12 May, suggesting that earlier settlement was unlikely.

After a period of high abundance between mid-June and beginning of August, the numbers of haddock in the demersal catches dropped. This decline could be due to the movement of growing juveniles into deeper waters (Fulton 1890, McIntosh 1897, Klein-MacPhee 2002), as haddock are generally associated with deeper waters (Klein-MacPhee 2002), and/or mortality of newly settled juveniles.

The abundance of juvenile haddock in the Stonehaven area was significantly higher (t-test; p<0.05) in 2005 than in the 2004 sampling season. As in the case of cod this finding must be taken with caution as only limited sampling took place in 2005. This information was, however, consistent with the data derived from the International Bottom-Trawl Survey (IBTS), the Scottish Ground Fish Survey (SCOGFS) and the English Ground Fish Survey (ENGGFS) (ICES 2007a,b). They all indicate that from the estimates of the 0-group haddock recorded in 2005 and 1-group haddock recorded in 2006, the 2005 year class was higher in abundance in the North Sea, particularly in the central and northern part, than year classes 2001-2004 (about 10 times higher than the average for these year classes). IBTS data show in 2005 what was referred to as a moderate size year class (in comparison to the 1999 high abundance year class) of 0-group haddock in quarter 3 (July-September), comparable in size to the 2000 year class, and in 2006 the higher numbers of 1-group haddock in quarter 1 (January-March). Data collected off Stonehaven in 2006 were not included in this analysis. Sampling in this year was carried out over very short period of time.

The size distribution of haddock in the pelagic and demersal catches confirms the pattern observed in their abundance. The inflow of the smallest juveniles was reported to take place

20 until mid-May in the pelagic zone (10 mm) and the second half of May in the demersal zone (30 mm). The smallest fish were observed in the demersal zone only for the initial first two weeks of sampling. After that period, a gradual increase in size of haddock was observed, which together with the absence of juveniles in the pelagic zone, indicated that there were no new settlers present. Juvenile haddock favoured deeper waters than cod. These depth differences were apparent from the very beginning of the settlement period, when cod favoured the shallower Site 1, while haddock settled in greatest numbers on Site 2, at intermediate depths. Haddock disappeared entirely from the shallowest site from the beginning of July. After an initial increase in abundance, towards the end of the season there was a slight decline in the numbers of juvenile haddock, especially at intermediate depths (Site 2). This decline could be due to mortality, a shift to structured habitat, movement of growing individuals out of the study area into deeper waters or a combination of the above. The size of haddock increased with depth, with largest fish found on Site 3, confirming that larger haddock show an affinity towards deeper waters, further away from shore, as they grow (Fulton 1890, McIntosh 1897, Klein-MacPhee 2002).

The results of this dietary analysis indicate that the settlement and following shift from pelagic to benthic type of prey was a gradual process, that occurred at the size range of 2.9 (±0.6) – 8.5 (±0.6) cm in case of whiting. The size that whiting could be considered settled according to the criterion of Bowman (1981) was estimated at 8.5 (±0.6).

Whiting had the most protracted settlement period among the species investigated. The first juveniles were sampled in the pelagic zone on 12 May 2004. The arrival of whiting occurred over an extended period of time, through most of the sampling season, with pelagic juveniles sampled between 12 May and 27 July. This was consistent with the general knowledge of the biology of whiting, which are known to have an extended spawning season, lasting until June- July (Hislop 1984). Therefore, the juveniles would be expected to arrive into the nursery area over a longer period of time, as it was the case in this study.

In the demersal catches whiting were found from 8 June 2004 until the end of the sampling season. The pattern of abundance indicated increasing numbers of the settlers through the season. This, combined with their pelagic abundance, points towards a very extended period of settlement for whiting, lasting from the beginning of June until the beginning of August.

21 The size distribution of 0-group whiting over the 2004 sampling season confirmed the patterns observed in the abundance data. Small newcomers were recorded in the pelagic catches through most of the settling season, right up to the disappearance of the juveniles from the pelagic zone. The demersal fish size distribution demonstrates the influx of small juveniles between the beginning of June and the end of July 2004. This period of settlement coincides with the period suggested by the abundance data. The presence of 40-60 mm juveniles in the pelagic catches could be an indication that whiting in this study were also undertaking vertical movements after settlement. Although settlement takes place over a relatively small size range, the settlement season for the whiting population is quite protracted (two months).

Throughout June whiting were found only on the shallowest site. After that time, their numbers increased rapidly on Site 2 and than on Site 3. On the shallowest site, the density of whiting reached a plateau by mid July at low densities. From mid July the highest densities of whiting were sampled on intermediate depths and towards the end of the sampling season on the deepest, most distant from shore site. Small whiting were present in the area from the beginning of June, several weeks after the first cod were caught, and for the first month were found only on the most inshore, shallow site. Throughout most of the sampling season the largest fish were found on the shallowest site, closest to shore.

In conclusion, differences in the timing of settlement among the three gadoid species investigated have been identified, with cod being the first species to commence settlement in the study area and whiting the last. There were also considerable differences in the duration of the process. Haddock settled in the Stonehaven area in one short pulse lasting approximately two weeks, while cod settled over longer pulse lasting approximately one month. Whiting displayed the most protracted settlement period, which is consistent with the spawning pattern of this species (Hislop 1984).

Juvenile cod, haddock and whiting in the Stonehaven area displayed patterns of distribution with depth and distance from shore that lead to species and size segregation in time and space. The analysis of the feeding ecology of cod, haddock and whiting showed major differences in dietary composition and little evidence of juveniles preying on each other (Demain et al. 2011), which further supports this conclusion. These factors all taken together can be presumed to result in niche separation and co-existence of these three species.

22 The above patterns lead to the conclusion that settlement of cod, haddock and whiting is a gradual process which may start with exploratory migrations towards the seabed, during which the juveniles start feeding on benthic prey and with bigger size they increasingly specialise in benthic feeding, although whiting continues to feed in the water column even at larger sizes. A number of authors referred to exploratory migration prior to actual settlement in the North Sea (Russell 1922, Bromley and Kell 1999). The presence in the demersal zone of very small juveniles still feeding on pelagic prey is consistent with that exploratory behaviour. The settlement size range calculated for cod in this study points out towards a transition to demersal life at smaller sizes than previously reported for North Sea cod (4.5- 6.5 cm, Robb and Hislop 1980; 4.0-8.0 cm, Bromley and Kell 1999). Also the settlement of haddock and whiting observed in this study took place at smaller sizes than those reported in the past (Heincke 1905, Robb and Hislop 1980). It is possible that earlier settlement is driven by the same fisheries induced evolution process which causes demographic changes in heavily fished populations (Wright and Trippel 2009). In exploited populations, fish display a high proportion of first time spawners (Wigley 1999, Morgan et al. 2003) and, as a consequence, have a narrower spawning season which can effect their reproductive success (Wright and Trippel 2009), and mature at a younger age (Trippel 1995). This earlier development could be carried over from the early-life stages and be evidenced by smaller settlement lengths, as well. However, to test this hypothesis would require further investigations into the timing and size at settlement on a much wider scale than in the present study. Alternatively, the differences between these settlement sizes and some of those reported in the literature may be the result of differences in the definition of settlement among authors, and/or the fact that most of those studies did not have available a data set comparable to that derived from the intensive series sampling regime carried out in this study. REFERENCES

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Number of analysed stomachs Type of analysis Year Cod Haddock Whiting

Numerical 2004 367 396 1103

2005 223 96 208

Gravimetric 2004 361 322 1023

2005 128 43 20

Table 1. A summary of the numbers of juvenile fish stomachs analysed by two different methods, sampled in 2004 and 2005. See text for a description of gravimetric and numerical analyses.

Type of sampler Year Cod Haddock Whiting 2004 10 152 96 Pelagic trawl 2005 2 0 2

2006 0 0 0

2004 455 822 2248 Demersal trawl 2005 190 95 209 2006 1 30 200 2004 N/A N/A N/A

Trap 2005 30 0 1

2006 27 0 0

Table 2. A summary of the total catches of cod, haddock and whiting over 2004-2006 sampling seasons by all types of gear.

model terms AIC value cod haddock whiting whiting* constant 49.9 127.5 144.1 136.9 site 51.4 130.0 143.6 139.6 period 42.1 109.9 144.3 138.3 period + site 43.8 112.1 142.5 140.6 period*site 47.8 116.1 140.9 138.9

Table 3. Model choice summary for pelagic data for cod, haddock and whiting. The best fitting models are indicated in red. Whiting* was an alternative model for whiting (see text).

model terms AIC value constant 198.2 site 196.3 depth 196.8 time 171.4 depth + time 168.0 site + time 164.0 site + depth 196.1 site + depth + time 163.7 depth*site + s(time) 162.8

Table 4. Model choice summary for cod caught by demersal trawls. The best fitting model is indicated in red.

model terms AIC value constant 283.4 site 265.0 depth 272.2 time 285.1 site + depth 267.0 site + time 266.0 site + s(time) 257.5 site + site:s(time) 265.1 site + s(time)+ depth 259.5

Table 5. Model choice summary for haddock caught by demersal trawls. The best fitting model is indicated in red.

model terms AIC value constant 295.5 site 298.9 depth 297.2 time 251.8 depth + time 252.2 site + time 252.8 s(time) 248.3 site + s(time) 247.1 depth + s(time) 248.0 site:s(time) 243.0 depth:site + s(time) 245.9 depth*site + s(time) 247.7 s(depth:site) + s(time) 253.0 site + site:s(time) 245.8

Table 6. Model choice summary for whiting caught by demersal trawls. The best fitting model is indicated in red.

34 model terms AIC value constant 1174.2 site 1159.9 time 1061.7 site + time 1031.8 site:time + time 1031.2 site:time + site 1033.2 site*time 1033.2

Table 7. Model choice summary for the length distribution of cod caught by demersal trawls in 2004. The best fitting model is indicated in red.

model terms AIC value constant 3382.8 site 3310.8 time 2243.1 site + time 2239.3 site:time + time 2238.8 site:time + site 2235.9 site*time 2235.9

Table 8. Model choice summary for the length distribution of haddock caught by demersal trawls in 2004. The best fitting model is indicated in red.

model terms AIC value constant 8022.4 site 7959.4 time 7512.9 site + time 7513.6 site:time + time 7513.2 site:time + site 7469.3 site*time 7469.3

Table 9. Model choice summary for the length distribution of whiting caught by demersal trawls in 2004. The best fitting model is indicated in red.

36 8

5 cod 2004 4 cod 2005

sqrt(no/h) cod 2006 3

0 90 121 152 183 214 245 day-of-year

Figure 1 a. Catches of cod in pelagic trawls in 2004 – 2006. Data points indicate square root transformed mean number of fish, standardised to number per hour effort, on the sampling day. Error bars indicate ±2SE.

Figure 1 b. Outcome of the mixed model for cod, showing changes in density with time (solid lines) on different sites (Site1 (12-26 m) – red, Site 2 (28-34 m) – black, Site 3 (36-46 m) – blue). Density is expressed as numbers of fish per hour of sampling effort. Dashed lines indicate ± 2SE confidence intervals. Dots represent data points.

38 20

12 haddock 2004 10 haddock 2005

sqrt(no/h) 8 haddock 2006

0 90 121 152 183 214 245 day-of-year

Figure 2 a. Catches of haddock in pelagic trawls in 2004 – 2006. Data points indicate square root transformed mean number of fish, standardised to number per hour effort, on the sampling day. Error bars indicate ±2SE. Note the scale differences between plots.

500

400

density (no/h) density 300

200

100

139 174 208 244

day-of-year

Figure 2 b. Outcome of the mixed model for haddock, showing changes in density with time (solid lines) on different sites (Site1 (12-26 m) – red, Site 2 (28-34 m) – black, Site 3 (36-46 m) – blue). Density is expressed as numbers of fish per hour of sampling effort. Dashed lines indicate ± 2SE confidence intervals. Dots represent data points.

40 16

10 w hiting 2004 8 w hiting 2005

sqrt(no/h) w hiting 2006 6

0 90 121 152 183 214 245 day-of-year

Figure 3 a. Catches of whiting in pelagic trawls in 2004 – 2006. Data points indicate square root transformed mean number of fish, standardised to number per hour effort, on the sampling day. Error bars indicate ±2SE. Note the scale differences between plots.

6000

5000

4000 density (no/h) density 3000

2000

1000

139 174 208 244

day-of-year

Figure 3 b. Outcome of the mixed model for whiting, showing changes in density with time (solid lines) on different sites (Site1 (12-26 m) – red, Site 2 (28-34 m) – black, Site 3 (36-46 m) – blue). Density is expressed as numbers of fish per hour of sampling effort. Dashed lines indicate ± 2SE confidence intervals. Dots represent data points.

41 Pelagic cod 2004 150 100 length(mm) 50

117 133 138 145 153 159 166 173 180 187 195 201 209 215 229

day-of-year

Figure 4. Length distribution of 0-group cod caught in the pelagic zone in the 2004 sampling season. Boxes indicate the 25th and 75th percentiles of all sizes measured. The thick line indicates median size.

Demersal trawls 2004 160 140 120 100 length(mm) 80 60 40 20

133 139 146 154 160 167 174 181 188 197 202 208 211 213 216 230 244 251

day-of-year

Demersal trawls 2005 160 140 120 100 length(mm) 80 60 40 20

133 139 146 153 160 167 174 181 188 198 200 208 211 213 216 230 244 251

day-of-year

43 Demersal taps 2005 160 140 120 100 length(mm) 80 60 40 20

133 139 146 153 160 167 174 181 188 198 200 208 211 213 216 230 244 251

day-of-year

Demersal traps 2006 160 140 120 100 length(mm) 80 60 40 20

133 139 146 153 160 167 174 181 188 198 200 208 211 213 216 230 244 251

day-of-year

Figure 5. Length distribution of 0-group cod caught in the demersal zone in the 2004-2006 sampling seasons. Boxes indicate the 25th and 75th percentiles of all sizes measured. The upper bars indicate the 10th, and the lower bars the 90th percentiles. The thick line indicates median size. Dots indicate outliers.

Demersal cod 2004-2006

160

140

120

100

length(mm)

133 139 146 153 154 160 167 174 181 188 198 200 202 208 211 213 216 244 251

day-of-year

Figure 6. Length distribution of 0-group cod caught in the demersal zone – combined data for 2004-2006 catches from demersal trawls and traps. Boxes indicate the 25th and 75th percentiles of all sizes measured. The upper bars indicate the 10th, and the lower bars the 90th percentiles. The thick line indicates median size. Dots indicate outliers. Coloured circles indicate trap catches – blue from 2005, green from 2006 and red from 2005 and 2006. On day 208 in 2006 only one cod was sampled by a demersal trawl.

Pelagic haddock 2004 150 100 length(mm) 50

117 133 138 145 153 159 166 173 180 187 195 201 209 215 229

day-of-year

Figure 7. Length distribution of 0-group haddock caught in the pelagic zone in the 2004 sampling season. Boxes indicate the 25th and 75th percentiles of all sizes measured. The upper bars indicate the 10th, and the lower bars the 90th percentiles. The thick line indicates median size. Dots indicate outliers.

46 A Demersal trawls 2004

160

140

120

100 length(mm)

133 139 146 154 160 167 174 181 188 197 202 208 211 213 216 230 244 251

day-of-year Demersal trawls 2005

B 160

140

120

100 length(mm)

133 139 146 153 160 167 174 181 188 198 200 208 211 213 216 230 244 251

day-of-year

C Demersal trawls 2006

160

140

120

100 length(mm)

133 139 146 153 160 167 174 181 188 198 200 208 211 213 216 230 244 251 day-of-year

Figure 8. Length distribution of 0-group haddock caught in the demersal zone in the 2004- 2006 sampling seasons. Boxes indicate the 25th and 75th percentiles of all sizes measured. The upper bars indicate the 10th, and the lower bars the 90th percentiles. The thick line indicates median size. Dots indicate outliers.

Pelagic whiting 2004

150

100 length(mm)

117 133 138 145 153 159 166 173 180 187 195 201 209 215 229 day-of-year

Figure 9. Length distribution of 0-group whiting caught in the pelagic zone in the 2004 sampling seasons. Boxes indicate the 25th and 75th percentiles of all sizes measured. The upper bars indicate the 10th, and the lower bars the 90th percentiles. The thick line indicates median size. Dots indicate outliers.

48 A Demersal trawls 2004

160

140

120

100

length(mm)

133 139 146 154 160 167 174 181 188 197 202 208 211 213 216 230 244 251 B day-of-year Demersal trawls 2005

160

140

120

100

length(mm) 80

133 139 146 153 160 167 174 181 188 198 200 208 211 213 216 230 244 251 day-of-year

49 C

Demersal trawls 2006

160

140

120

100

length(mm)

133 139 146 153 160 167 174 181 188 198 200 208 211 213 216 230 244 251 day-of-year

Figure 10. Length distribution of 0-group whiting caught in the demersal zone in the 2004- 2006 sampling seasons. Boxes indicate the 25th and 75th percentiles of all sizes measured. The upper bars indicate the 10th, and the lower bars the 90th percentiles. The thick line indicates median size. Dots indicate outliers.

A 1.0

0.8

0.6 prey proportion

0.4

0.2

0.0

5 10 15

length(cm)

1.0

0.8

0.6 prey proportion 0.4

0.2

0.0

5 10 15

length(cm)

Figure 11. Proportion of benthic prey items in the diet of 0-group cod, by (A) prey weight and (B) numbers. Data points represent percentage of benthic prey in individual fish. Black solid line is the logistic regression model fit. Black dotted lines indicate ±2SE confidence intervals. Red solid line indicates calculated l50 with ±2SE confidence intervals indicated by red dotted lines.

A 1.0

0.8

0.6 prey proportion 0.4

0.2

0.0 5 10 15

length(cm)

B 1.0

0.8

0.6

prey proportion

0.4

0.2

0.0

5 10 15

length(cm)

Figure 12. Proportion of benthic prey items in the diet of 0-group haddock, by (A) prey weight and (B) numbers. Data points represent percentage of benthic prey in individual fish. Black solid line is the logistic regression model fit. Black dotted lines indicate ±2SE confidence intervals. Red solid line indicates calculated l50 with ±2SE confidence intervals indicated by red dotted lines.

1.0

0.8

0.6

prey proportion 0.4

0.2

0.0 5 10 15 length(cm)

Figure 13. Proportion of benthic prey items in the diet of 0-group whiting, by (A) prey weight and (B) numbers. Data points represent percentage of benthic prey in individual fish. Black solid line is the logistic regression model fit. Black dotted lines indicate ±2SE confidence intervals. Red solid line indicates calculated l50 with ±2SE confidence intervals indicated by red dotted lines.