School of BioSciences - Theses
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ItemTrawling for the unknown: An investigation of the impacts of harvest and oceanic warming on the life-histories of fish( 2021)Fishing is globally important as humans rely on fish for 17% of their total protein intake and millions of people are directly or indirectly dependent on fisheries as a source of income. Fisheries harvests do, however, impose a number of impacts on fish populations. Harvest can cause demographic truncation (loss of large and old individuals), as well as reducing the reproductive capacity of populations and modify ecological interactions. Furthermore, fishing is inherently size-selective, where the preferential catch of certain phenotypes using different gears can combine with relatively high rates of fishing mortality to drive potentially rapid evolutionary responses in populations (called fisheries induced evolution or ‘FIE’). As a result of fishing, many harvested stocks are expressing a faster ‘pace-of-life’, characterised by faster growth, earlier maturation, and increased relative reproductive investment. Concurrent to fishing activity, the world’s oceans are warming at unprecedented rates which is having significant impact on the physical environment experienced by fish. Importantly, warming is thought to drive a similar increase in the pace-of-life expressed by fish via direct physiological and indirect temperature-dependent processes. This can be seen in almost ubiquitous shifts to faster growth, earlier maturity and smaller adult size, a phenomenon called the temperature size rule (TSR). Warming is also impacting fish populations through factors such as increased mortality, range shifts and increased incidence of hypoxic conditions. Our understanding of how the effects of harvest and warming may combine to impact on fishes, and influence their subsequent recovery after initial impact, is poor. An improved understanding of the combined impacts of fishing and warming oceans and the mechanisms underpinning these, as well as heuristic, cost effective ‘monitoring’ tools are needed if we are to properly manage fish stocks into the future. In this thesis, I tested a recently published biphasic growth model that attempts to estimate age at maturity from commonly collected size-at-age data (Chapter 2). Statistical techniques such as these could be utilised as cost and time-effective fishery monitoring tools. I used data from multiple North Sea stocks to parameterise the biphasic model and found that the method performed poorly given the data. This poor performance was primarily due to the lack of temporal continuity and representation of older individuals which decreased the ability of the technique to model growth and thus estimate maturity. I suggest that future fisheries monitoring should include improved data collection across all age and size classes so that we can adequately utilise potentially cost-effective statistical techniques to estimate key life-history parameters. I then ran a multi-generational selection experiment that applied interactive treatments of warming (26 C ‘control’ and 30 C ‘warmed’) and realistic fisheries selection (random harvest ‘control’, ‘sigmoidal’ and ‘Gaussian’ selection) to 18 populations of the tropical freshwater zebrafish (Danio rerio). My sigmoidal treatment was designed to mimic trawl fisheries, where the largest individuals are harvested, and my Gaussian selection treatment simulated gillnet or slot limited fishing, where the midrange of sizes were affected by fishing. Selection was applied to six generations, where I followed responses across these ‘F’ generations as well as two common garden generations where selection was relaxed. Studying responses under common garden conditions reveals information as to the nature of responses, where plastic, maternal and evolutionary processes can be teased apart. In Chapter 3, I tracked the expression of early life-history traits in populations exposed to fishing and warming and found that recruitment collapsed after four generations of warming treatment. Moreover, temperature interacted with fishing such that sigmoidal harvested populations had the lowest recruitment rate. This interaction indicates that the removal of large fish individuals through harvesting can exacerbate warming-induced recruitment collapse. Preserving size diversity in fish populations is important to help increase the resilience of fisheries to the impacts of warming. In Chapter 4, I tested two alternative explanations of the temperature size rule (TSR), which describes the phenomena of increased juvenile growth, early maturity and smaller adult sizes at higher temperatures in ectotherms. Debate centres around whether TSR is the result of warming-induced physiological constraints on growing to larger sizes, or an adaptive outcome stemming from temperature-induced energy allocations. I found evidence that the metabolic rates of fish held at elevated temperatures acclimated after three generations. This result indicates that the commonly proposed physiological limitation mechanism could not solely explain TSR. Instead, I found evidence that populations adjusted reproductive investment via earlier maturation and higher early investment in reproduction, suggesting that TSR could be explained by shifts in resource allocation. These results have implications for models used to predict warming impacts in our oceans. Finally, I analysed trends in the expression of multiple juvenile and adult life-history traits through generations (Chapter 5). Here I found that warming and sigmoidal fishing treatments led to the largest reductions in adult body sizes. In contrast, fisheries selectivity that preserved large individuals allowed warmed population to maintain body sizes similar to those in the control temperatures. Temperature impacts on life-history traits recovered rapidly in common garden generations, whereas the legacy of fisheries selection remained. These findings suggest that even five generations of realistic fishing pressure can have long term impacts on populations, affecting their response to management interventions. Together, my work advances our understanding of how harvested populations respond to significant contemporary and future stressors and provides valuable insights into the future sustainable management of fisheries resources.