Fisheries sustainability in the presence of predation by marine megafauna

Tutkimustuotos: OpinnäyteVäitöskirjaArtikkelikokoelma


Overexploitation is still a leading problem of many commercially targeted fish species. In addition to the high harvest rates and increasing biomass removals, harvested marine ecosystems have become a stage for the dynamic interplay of evolutionary and ecological processes. Removal through size selective fishing gear can cause negative pervasive effects on individual as well as population level. Observations of the individual phenotypic traits show a general trend of decreasing size and age at maturity that can have further negative effects on fecundity and population productivity. As these phenotypic changes become heritable (i.e., fisheries-induced evolution or FIE), this can further diminish the fish available to fisheries and render future fishing yields unsustainable.

Current management requires additional measures to include avoidance and detection of evolutionary changes. In order to understand which fishing objectives precede evolutionary change in individual traits, in my thesis I explored how different fishing strategies of the European hake (Merluccius merluccius) fishery reflect on ecological and evolutionary processes. While management focusing on the protection of juvenile fish can minimise the negative ecological impact of fishing, it increases the potential for evolutionary change in fish phenotypic traits. In contrary to this, fishing mortality targeting a wider range of age–size classes avoids evolutionary shifts in individual traits, however such fishing strategy demonstrates higher biomass removals.

In the wild, fisheries continuously interact with other predators, such as marine mammals, which can prey upon the same fish species or stock. The impact of these direct and indirect biological interactions between the marine mammals and fisheries is harder to detect and quantify, especially in synergy with other natural or anthropogenic stressors. In the context of fisheries-induced evolution, changes observed on an individual and population level caused by fisheries will also affect the prey size selectivity and prey availability to natural predators. My synthesis of recent research and findings on marine mammal–fisheries biological interactions demonstrates the need for improvement on data regarding marine mammal dietary and energetic requirements as well as their representation in model-based approaches. Moreover, combining different sources of knowledge about marine mammal–fisheries competition can aid to better quantify fish mortality caused by predation. Subsequently, this information would improve the fish stock assessments and provide insight on a sustainable window of opportunity to catch fish for fisheries and natural predators.

Thus far, attempts to quantify predation and fish availability for fisheries and natural predators exist through studies using mainly ecosystem and fisheries models. To explore how predation and fisheries shape and direct individual as well as population parameters, I have used an individual-based model to simulate hake growth trajectories with regards to its own biological characteristics. As an individual grows, its life history is formed by ecological and evolutionary processes which also take into account the reproductive cost of survival and sexual size dimorphism (SSD). With co-evolved interactions between hake and the bottlenose dolphin (Tursiops truncatus) as the predator, fishing is introduced through a limited time period in order to observe prey recovery and resilience on an individual and population level. Although different types of predation give insight to discrepancies in the intensity of predation mortality, mere presence or absence of predation determines the projected values reached by prey individual and population parameters. Moreover, the joint effect of predation and fishing reveal contra-intuitive trends in hake individual traits and population parameters. The combination of duration and intensity of both size-selective removals, predation type and SSD determine the potential for persistent phenotypic and demographic changes after a period of overexploitation. Additionally, not all individual traits are equally susceptible to fisheries-induced evolution where the accountability of SSD and predation type can play a critical role. While fisheries remain the most detrimental source of mortality and size-selective removal for the harvested species, the indirect effects of fishing intensity diminish predator survival, thus having direct implications for top predator conservation. In conclusion, increasing the biological realism of the targeted species and incorporating different predation types with respect to evolutionary processes provide a more holistic approach to fisheries management: as it helps to avoid potential FIE and an overestimation of fish available to fisheries that can prevent top predator collapse. This will, ultimately, lead to a more ecosystem-based management with sustainable harvest rates and optimised fishing effort as well as the minimal cascading effects of size-selective removals.
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