1 Federal Research Centre for Fisheries Institute for Sea Fisheries, Hamburg Hans-Joachim Rätz Josep Lloret Institut de Ciències del Mar, Barcelona Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland

2 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 1. Introduction 2. Growth variation based on long term trends in size at ages 4 and Temperature effect 2.2 Fishery selection effect (fishing mortality) 2.3 Stock size effect (density) 2.4 All effects 3. Recruitment variation based on long term trends in strength of age group Temperature effect 3.2 Spawning stock size effect 3.3 Recruitment off Iceland 3.4 All effects 4. Summary

3 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 1. Introduction Cod in Greenland waters Atlantic cod (Gadus morhua) is described a common species in the Greenland fauna, although reaching here its ecological northern boundary (Hansen, 1949; Buch et al., 1994). Thus, Greenland cod has often been studied to analyse and quantify environmental effects on stock production determined from variation in natural mortality, growth and recruitment. The cod stock off Greenland has been commercially important. From 1955 to 1970, landings varied around t/a. Relation between the Greenland and Icelandic cod stocks: Spawning grounds off southwest and southeast Greenland and southwest Iceland (blue areas). Egg, larval and O-group drift from Iceland to Greenland (dashed arrows) Homing migration of adult fish to Iceland (solid arrows)

4 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 1. Introduction Data used Length at ages 4 and 5 Since 1955, cod length and age data were obtained from German commercial cod catches off East and West Greenland and from annual scientific bottom trawl surveys in autumn since The sampling program from 1955 until 2002 comprised a total of 222,426 individual length measurements and age determinations covering the age range from 0 to 31 years. Only mean length at age values by area, year and quarter based on at least 30 individual age determinations were considered sufficiently representative and used in the present growth investigations. Such filter limited the data available to time series of mean length at ages 4 and 5 years only based on 52,973 age determinations, as only these two age groups are found consistently covered by the sampling over the entire investigation period. Mean lengths at ages 4 and 5 were corrected for area and quarter effects. Temperature Water temperatures on top of the Fyllas Bank off West Greenland were measured by the Greenland Fisheries Research Institute during mid-June of each year since Buch (1984) showed that the temperature trends in neighbouring areas and periods were similar to those on Fyllas Bank due to the process of advection and solar heating. The values considered in the present analyses are the average water temperature of upper 50 m adopted from Buch and Ribergaard (2003). The most recent decade was distinguished by about average or above average water temperatures during June on top of Fyllas Bank. For each cohort, the mean temperature since age 3 was used to account for any cumulative effect.

5 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 1. Introduction Data used Stock parameters The present study is a revision of earlier investigations, which were constrained by data until 1992; since then, 10 new data points are made available mainly from survey data collected. Assessment update from the last assessment carried out by ICES in 1996 with 1992 as the terminal year. Assessment update (XSA) is consistent with both historic and most recent information from surveys and CPUE analyses. In addition to recruitment estimates at age 3 we used SSB, 3+ stock in numbers and Fref5-8 in the models. We also adopted recruitment estimates at age 3 of the Icelandic cod stock to analyse the effect of year class strength off Iceland on the recruitment success in the Greenland cod stock (ICES 2003). Fishing mortality was highly variable because of high variation in stock size and effort and dropped from high values to low after the termination of the directed fishery in SSB and recruitment remained very low over the last decade.

6 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 2. Growth variation based on long term trends in size at ages 4 and Temperature effect Length at ages 4 and 5 Highly significant positive temperature effect, but only about 15 % explanation of the variation observed.

7 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 2. Growth variation based on long term trends in size at ages 4 and Fishery selection effect (fishing mortality) Length at ages 4 and 5 Insignificant fishery selection effect.

8 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 2. Growth variation based on long term trends in size at ages 4 and Stock size effect (density) Length at ages 4 and 5 Significant negative stock size effect on length at age 4 but not on age 5.

9 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 2. Growth variation based on long term trends in size at ages 4 and All effects Length at ages 4 and 5, significant multiple models explaining about 40 % of the variation observed. Highest regression weight for temperature effect, only 1 outlier outside 95 % confidence prediction intervals, the low sized 1985 year class in both models. Models capture recently increased size at ages 4 and 5 as driven by high temperatures, low stock size and low fishing mortality.

10 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 3. Recruitment variation based on long term trends in strength of age group Temperature effect Temperature has a significant positive effect on year class strength at age 3, explaining only about 13 % of the observed variation. Since the mid 1950s, no rich year classes were observed when temperatures of the upper 50 m fell below 1°C.

11 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 3. Recruitment variation based on long term trends in strength of age group Spawning stock size effect SSB has a significant positive effect on year class strength at age 3, explaining about 48 % of the observed variation.

12 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 3. Recruitment variation based on long term trends in strength of age group Recruitment of Iceland Recruitment at age 3 off Iceland has a significant positive effect on year class strength at age 3 off Greenland, explaining only about 12 % of the observed variation. Since the mid 1950s, no rich year classes were observed off Greenland when recruitment off Iceland fell below 150 Mill.

13 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 3. Recruitment variation based on long term trends in strength of age group All effects Recruitment at age 3, multiple effects explain about 60 % of the observed variation. SSB gains by far the highest exploratory weight, followed by the temperature effect. The recruitment off Iceland has a low but significant positive effect on year class strength, indicating a rare contribution to Greenland recruitment, especially during the recruitment failure of the last 2 decades. The rather wide prediction interval at 95 % confidence prevents precise recovery predictions.

14 Long-term variability of growth and recruitment of cod (Gadus morhua) off Greenland Hans-Joachim Rätz and Josep Lloret 4. Summary Variability of growth Growth variation was estimated from mean lengths at ages 4 and 5 years for Atlantic cod off Greenland based on samples taken from commercial and scientific catches comprising 52,973 individual age determinations since Significant growth checks occurred during the 1960s and from the mid 1970s until mid 1980s. During the most recent decade analysed, from 1993 until 2002, the annual growth rates of cod in Greenland waters were consistently among the highest recorded. Multiple linear regressions revealed significant positive temperature effects on growth of both age groups and a significant negative stock density effect on length at age 4. Exploitation intensity was found statistically insignificant to explain inter-annual variation in growth. Variability of recruitment During the period (assessment update includes most recent decade), the number of recruits at age 3 years was significantly correlated with the spawning stock biomass of the Greenland cod stock, June water temperature on top of Fyllas Bank (West Greenland) and recruitment off Iceland. All factors positively affected the number of recruits off Greenland and explained 60 % of the observed variation. The spawning stock size was by far the most important factor contributing to the recruitment variability while June water temperature below 1°C and recruits off Iceland below 150 million never coincided with high year class strength in Greenland during the past 50 years. Former conclusions regarding a close relation between the Greenland and Icelandic cod stocks might need to be reconsidered in the light of a continued recruitment failure in Greenland waters over the 2 most recent decades, despite the improved environmental conditions and about average recruitment strength of the Icelandic stock.