Modelling the potential impacts of fisheries on ecosystem dynamics using tropho-dynamic models

Abstract

Despite many studies of the effects of fisheries on ecosystem dynamics, the consequences of removing food subsidies or predators from marine ecosystem are still unclear. In this thesis, I addressed two key areas in relation to our understanding of how fisheries affect ecosystem processes: (i) food subsidies and (ii) the effects of removal of predators. These two specific areas were selected as they have a potential of modifying ecosystems, and may lead to loss of the essential goods and services ecosystems provide. There are several threats to marine ecosystems, one of them being overfishing. The impacts of overfishing include changes in biological assemblages and modification of ecosystems. The Ecosystem Based Management (EBM) in fisheries has been widely advocated as it encompasses interactions within the ecosystem; and ecosystem models, which are able to address various scientific questions in ecosystems, have been widely used as a tool for advancing this process. In my thesis, I aimed to understand the effects of discarding and selective overfishing on ecosystem dynamics. Studies on the effects of discards on ecosystem dynamics have produced variable results, which creates uncertainty in concluding how ecosystems respond to discards, and presents a management challenge. It is still unclear whether discards should have similar impacts on all ecosystem types. To address this problem, in Chapter 2, I performed a global meta-analysis of 23 studies that were manipulated to explore the effects of discards on ecosystem functions (productivity, respiration and consumption). I examined the following variables: predator biomass, predator catch, total catch, total primary production/ total respiration (TPP/TR), system omnivory index (SOI) and primary production required for the catch (PPR catch) from the studies, with 9 explanatory covariates: area, functional groups, stanza groups, publication type, latitude, study duration, discards biomass, ecosystem type and fleet size. I found that presence or absence of life history age-class structured data (stanzas) gave divergent responses. All variables, except predator biomass, showed an increase in the presence of food subsidies. Explanatory covariates that provided the greatest explanatory power in the models were stanza, ecosystem type and publication type. I concluded that inclusion of stanzas could give different model predictions and presence of discards may increase ecosystem functions. In Chapter 3, I describe the methodology for the models used in Chapters 4 to 6. The issue of discards has drawn global attention in the recent past, so in Chapter 4, I addressed this problem by exploring the consequences of gradually removing and abruptly banning discards using ecosystem models. Fisheries discards are a major source of Predictable Anthropogenic Food Subsidy (PAFS) in commercial fisheries. PAFS impact ecosystems by modifying ecological processes and food webs. I found that PAFS increase food pathways of opportunistic scavengers. When PAFS were reduced gradually, scavengers were able to switch their prey. From this work, I recommended gradual reduction of PAFS to allow species exploiting PAFS to adjust to reduction of food subsidy. Predators play a key role in maintaining the structure and function of ecosystems. Removal of predators resulting from overexploitation could have detrimental effects on ecosystems. In Chapter 5, I examined the consequences of simultaneous removal of predators from different trophic levels of a subtropical food web. I explored the effects of removal of top predators, meso-predators and small predators in Moreton Bay where different types of fishing remove different components of the food web. I found that when crabs were fished out, macrobenthos and prawns increased. Removal of pelagic fish resulted in an increase of jellyfish, while removal of sharks resulted in an increase of major groups, with the highest increase in the pelagic fish. When all predators were fished out, the low trophic level species increase in biomass. The findings emphasized the critical role that predators play in maintaining healthy ecosystems. The increased incidences of jellyfish blooms in many parts of the world have been related to overfishing. Overfishing results in decline of fish that prey on or compete with jellyfish. In chapter 6, I simulated overfishing of jellyfish in Moreton Bay model. As it was anticipated, overfishing of jellyfish impacted negatively on pelagic fish which prey on jellyfish. In summary and conclusion (Chapter 7), discards increase ecosystem functions (productivity, respiration and consumption); but a gradual ban rather than a complete ban on discards is recommended where mega-fauna have become dependent on discards as a food source. It is necessary to regulate overfishing of predators in order to maintain the structure and health of marine ecosystems. Jellyfish blooms related to overfishing may be reduced by fishing jellyfish, but in some cases may have an effect on major top predators, in such cases, fishing jellyfish may not be a desirable management option.