Recruitment processes in West Greenland waters : with special focus on Greenland halibut (Reinhardtius hippoglossoides, W.)

Abstract

The topic of this thesis is recruitment processes in Greenland Halibut (Reinhardtius hippoglossoides) in West Greenland waters. The focal point is Greenland halibut’s early life history from oocytes in the female ovary - to larvae living in the pelagic - to juveniles on the sea bottom. These early life stages are examined in four studies: Ovary and oocyte development in adult female Greenland halibut was studied in Davis Strait, Disko Bay and Baffin Bay. The objective of this study was to track and compare ovary development from winter to autumn from the inshore fjords of Disko Bay, Baffin Bay and the presumed spawning area in Davis Strait. Results showed that maturity was much more progressed in Davis Strait compared to the two other areas. Furthermore, almost all adult female fish in Davis Strait showed signs of maturation while only a fraction of the larger females (>80 cm length) were mature in Disko and Baffin Bay. A large proportion of the Greenland halibut in Disko and Baffin Bay did not appear to begin the maturation cycle until very late in their life history and/or were repeat spawners with a multiyear maturation cycle. Thus the observations from Disko and Baffin Bay support the theory that Greenland halibut can have a prolonged adolescent phase. The result from the study also strongly indicated that Davis Strait was the only major spawning area in West Greenland. Larvae distribution and growth of Greenland halibut and Sandeel (Ammodytes sp.) was studied across the West Greenland shelf. The objective of this study was to analyse spatial distribution and growth during three periods May, June and July and relate these to cross shelf variability in biological and hydrographical conditions in order to identify areas of special importance for larval growth and survival. Results showed that in May small Greenland halibut larvae, some still with small remains of the yolk-sac, were primarily distributed offshore in Davis Strait. Their distribution coincided with their prey resources while some vagrant larvae were dispersed to areas with less food, which likely resulted in higher mortality rates. Consequently, the emergence of first feeding Greenland halibut larvae matched their prey in time and space which is believed to be crucial for survival success at this critical stage in their development. From June to July Greenland halibut larvae gradually shifted their distribution from the bank slopes to the deeper parts of the slope near the shelf break. The lack of clear and well-defined frontal zones made it difficult to evaluate the hydrographical processes that had significance for larval distribution patterns. Greenland halibut larvae did, however, show preference for stratified water masses. Growth analysis of Greenland halibut larvae was only possible for July and showed higher growth was observed at stations positioned in frontal zones suggesting that growth was significantly influenced by the level of frontal activity. The study also included distribution and growth comparisons with sandeel. The larvae of sandeel was considerably more associated to the banks and bank slopes than Greenland halibut larvae in all three months (May-July), however in June, the two species coexisted considerably in the same areas of the cross shelf. The growth analysis showed that sandeel and Greenland halibut larvae showed opposite responses to some hydrographical conditions and prey abundance. Sandeel growth decreased with increasing frontal activity and increased with increasing prey biomass. The later observation suggests that sandeel growth was limited by food availability. Cross shelf variation in growth in Greenland halibut larvae showed that enhanced growth was observed at some stations where they were most abundant. Overall, however, there was no clear association between larval growth and larval abundances for neither Greenland halibut nor sandeel larvae. The observed patchiness in larval distribution could therefore also be a result of retention mechanisms and / or predation mortality. The feeding ecology of pelagic Greenland halibut and sandeel larvae was studied across the West Greenland shelf. The aim of this study was to analyse and compare feeding ecology of coexisting Greenland halibut and sandeel larvae. Diet analysis and prey preference considerations were used to assess spatial variation in prey availability and feeding. The hypothesis was that feeding conditions are optimal in central areas of fish larval distribution, and that timing of larval emergence is synchronised with prey availability. The study showed that for both species copepods were the main prey items and that absolute size of preferred prey increased during larval ontogeny. However, preferred copepod size in relation to larval length differed markedly between Greenland halibut and sandeel. In Greenland halibut the relative size of the prey declined during growth of the larvae while for sandeel the relative size of prey remained constant at a level of 2.7% of larval length. This led to a reduction in prey niche overlap between the two species as Greenland halibut and sandeel larvae increased in size. The available prey copepod biomass differed distinctly across the shelf area. In May, Greenland halibut prey density was highest in the off-shelf area in Davis Strait. In June and July, the prey rich areas for both species were mainly located on the slopes of the banks and at the shelf break area. In these areas gut fullness was higher than in neighbouring areas, suggesting that the food resource could be scarce. The feeding ecology of Greenland halibut and sandeel could explain why larval abundance indices for the two species historically have shown opposite responses to annual environmental conditions and total zooplankton occurrence. Drift and growth of Greenland halibut from eggs throughout the pelagic larval stage was tracked from sub-population spawning areas in Davis Strait, Baffin Bay, East Greenland and West Iceland by combing drift- and individual-based egg and larval temperature dependent growth models. Model results were compared to survey results and the settling process was followed at Store Hellefisk Bank in West Greenland. The aim of this study was to evaluate spatio-temporal distribution of small first feeding and large settling larvae, and the between metapopulations. Results showed that egg and larvae can drift for long distances and that the exchange of individuals among the geographically-separated sub-populations could be important for metapopulation dynamics. Larvae from the spawning area in Davis Strait were primarily (>60%) transported to Canadian waters. Only larvae from the eastern part of the spawning area in Davis Strait remained in West Greenland. All larvae from the East Greenland spawning area were transported south of Cape Farewell and to West Greenlandic (82%) or Canadian (18%) waters. From the West Icelandic spawning areas larvae either, depending on the spatial location on the emergence first feeding larvae, drifted to East Greenland (45-53%), West Greenland (19−34%) or Northern Iceland (21−28%) or remained (>98%) in Iceland waters. In Baffin Bay low water temperatures resulted in very slow development rates and eggs probably never hatched. The surveys showed that in West Greenland larvae settling started in August and continued in September but the settling peak period could not be determined. The study in the two areas of Hellefisk Bank showed that juveniles of age 1 and 2+ almost exclusively occurred in one area while the 0-group was equally distributed in both areas. In the area of high juvenile age 1 and 2+ abundance the condition and stomach fullness was significantly higher than in the low abundance areas. Nursery ground processes within the first year clearly restricted the successful nursery area to a more specific bottom habitat types. The concentrating of juveniles into specific nursery areas implies that juvenile densities may approach the carrying capacity of their habitats in years when settlement is high, which would dampen the annual variability in year class strength of Greenland halibut.