| Summary: | Across the period 1988-2008, the most abundant demersal species were cod, redfish, Northern shrimp and Greenland halibut accounting, as an average, for the 83.5% of total index of biomass every year. The analyses of biomass indices showed that the demersal community experienced notable variations across the period 1988-2008, due to changes in biomass of most of the 67 demersal populations studied. The Dynamic Factor Analysis (DFA) identified common trends in the trajectories of the 31 most abundant species in the demersal community. This suggests that the dynamics of the demersal species in the Flemish Cap are interconnected, and can be summarized by a few common patterns. The explanatory variables considered in the analyses appeared to be consistently important for the population biomass dynamic of these species. Water temperature, along with predation and fishing mortality were significant drivers of the Flemish Cap demersal community. The abundance of the less common demersal species was related with water temperature, with a transition in the species composition between cold and warm periods. Changes in the demersal community were globally registered in the diversity indexes and the Abundance Biomass Comparison (ABC) method, with notable variations in the relative location of biomass and abundance kdominance curves. The size based indicators showed marked declines in the size structure of the fish demersal community. Parallel to these changes in the demersal community, since 1993 important variations in feeding habits for the most important fish species were observed. First, strong variations in feeding habits with size were found in most fish species and hence, biological species were split into trophic species. These trophic species belonged to four different trophic guilds, the bentho-pelagic invertebrate feeders, the benthic invertebrate feeders, the pelagic invertebrate feeders and the piscivorous guild. Not only intra-guild but also inter-guild common trends were found. The dominant common trend was the increase on shrimp consumption for most trophic species, although in the piscivorous guild the consumption of redfish also presented an increasing trend. Parallel to this, intra-guild common trends toward the decline on consumption of their usual preys like ophiuroids, hyperiids and copepods were also detected. The variables accounting for common trends were mainly the abundance of prey species, the intra-guild competition and the oceanographic conditions. These common trends led to a higher overlap in feeding habits at the end of the study period. These results highlight the importance of trophic interactions in management decisions. The importance of key preys like Northern shrimp or juvenile redfish for other commercial species would need to be considered when establishing fishing quotas, i.e. a multispecies approach to fisheries management instead of a monospecific approach. On the other hand, variations in feeding habits with fish length highlight the necessity of considering the demographic structure of both prey and predators when including trophic interactions in management decisions. Contemporaneously with changes in abundance of the Flemish Cap cod, changes in reproduction, growth and condition were detected. The high fishing mortality registered in the late 1980’s and early 1990’s contributed very importantly to the observed decrease in cod biomass. A genetic change toward earlier age and smaller size at maturation was found already in the 1980’s but especially in the early 1990’s cohorts. The decrease in Female Spawning Stock Biomass (FSSB) as consequence of the steep decline of the population, in conjunction with the rejuvenation of the reproductive stock, led to the decrease in the Total Egg Production (TEP). The high correlation of TEP with the recruitment during this period supports that the decrease in the SRP was largely responsible of the recruitment failures since mid 1990’s. However, the low temperatures recorded between 1989 and 1997 may have also lead to unfavorable conditions for cod recruitment. Cod fishing still remained in the Flemish Cap until 1996, which in conjunction with the absence of good recruitments, was the final blow contributing to the collapse. Since then density-dependent processes led to an increase in condition and growth, favoring earlier maturation by phenotypic plasticity and a growing FSSB. Although not studied in this thesis, it is probable that during this period the higher fish condition would led to an increased relative fecundity, producing an increase in the TEP. In the absence of fishing pressure, a higher SRP together with improved feeding conditions and higher temperatures probably favored the good recruitment events observed since 2005 and the recovery of the stock. With previous information, an extended food web model, including the 14 most important fish demersal species and their main preys was developed. A simplified conceptual model with cod, redfish, shrimp and Greenland halibut trophic interactions, including all the main drivers for population and community dynamic was also created. This included inner drivers like population structure and abundance, growth, condition and SRP, and external drivers like oceanographic conditions, species interactions (mainly predation but also competition) and fishing. With this theoretical model and the information from previous analysis a description of the ecological functioning of the Flemish Cap demersal community was developed. Fishing activity in conjunction with predation and oceanographic conditions were considered the main drivers inducing changes in the population structure of various species through mortality of larval, juvenile and adult stages. These changes produced variations in trophic interactions between species, and density-dependent processes affecting to growth and condition. All these factors strongly affected to the SRP of populations, which in turn affected to population structure. Under this scenario, fishing on cod and redfish under adverse environmental conditions for recruitment and low SRP produced an imbalance in the ecosystem that led to the increase of shrimp and Greenland halibut by the release of predation and competition respectively. Redfish stocks benefited from the low biomass of a capital piscivorous like cod, showing excellent recruitments in a period of favorable oceanographic conditions. The higher availability of shrimp and redfish preys produced an increase in the SRP of cod, which favored the recovery of the cod stock since 2005. The increasing predation of a growing cod stock produced the decline of the redfish stock since 2008, while the Northern shrimp stock decline was ascribed to the increasing predatory pressure from both cod and redfish in conjunction with a very high fishing pressure. This conceptual model developed to study the ecological functioning of the demersal community in Flemish Cap supposes an integrative approach that represents an important step away from the traditional view of single species management in Flemish Cap. This type of contributions constitutes major steps towards a new framework for fisheries management that incorporates theoretical background on the functioning of marine ecosystems. Achieving a sustainable fishery requires to focus on sustaining relationships between species, which includes fishery within complex evolving ecosystems. Fisheries management should maintain these relationships stable and robust within a resilient ecosystem.
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