Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Department

Fisheries, Animal and Veterinary Science

First Advisor

Austin Humphries

Abstract

Access to markets and globalization has fueled the expansion of fishing pressure in coral reef fisheries and has consequently led to overfishing on many reefs around the world. Overfishing has profound consequences for reef ecosystems and the people that rely on reef resources for nutrition and income. Collecting fisheries-dependent data required for traditional stock assessment methods that guide management is often infeasible for coral reefs due to the small-scale, dispersed, multi-species, and multi-gear nature of these fisheries. In addition, coral reefs are concentrated in developing countries that often have inadequate capacity for management and enforcement, which limits the effectiveness of the default management strategy for marine ecosystems around the world – marine protected areas. However, fishers on coral reefs use a variety of fishing gears and generally favor gear-restrictions over spatial closures. Thus, the aims of this dissertation were to investigate the effects of fishing pressure and habitat condition on reef fishes, quantify the selectivity of fishing gear types (i.e., hook-and-line, nets, and spears) for functional groups and size classes of reef fishes, and generate a fisheries model to observe population dynamics of reef fishes under various gear-restriction management scenarios.

Chapter 1 is an overall introduction for the dissertation that provides background information for Chapters 2, 3, and 4. The general importance and scale of coral reef fisheries globally is outlined, and detailed information on fisheries-independent indicators (specifically size-spectra) and gear-restriction management are provided in this chapter.

In Chapter 2, we investigate the effectiveness of a fisheries-independent indicator – size spectra slope – for detecting the effects of fishing and habitat condition on reef fishes. Size spectra slopes quantify the relationship between relative abundance and size of organisms. Size spectra can be useful for the assessment of coral reef fishes because fish life-history characteristics are related to fish size and, thus, can represent the flow and distribution of energy through food webs. We quantified benthic composition and size spectra of coral reef fishes using underwater visual surveys across three regions of Indonesia. We found that total fish biomass (proxy for fishing pressure) was the best predictor of size spectra for reefs fishes. Also, we found an interaction effect between fishing pressure and structural complexity such that structural complexity decreased as fishing pressure increased. Thus, suggesting that fishing pressure was associated with habitat degradation. Biomass and structural complexity were also the best predictors of size spectra slopes when carnivores and herbivores were analyzed separately, but carnivores had a sharper decrease in size spectra (i.e., fewer small fish) as fish biomass decreased. Our results suggest that size spectra slopes could be an efficient method for assessing coral reef fish populations.

In Chapter 3, we quantify the selectivity of hook-and-line, net, and spear fishing in Wakatobi National Park in Southeast Sulawesi, Indonesia. We determined the catch composition using fisheries-dependent data, and developed selectivity indices by using the Manly-Chesson selectivity equation. The Manly-Chesson equation allowed us to calculate selectivity as fishes caught (fisheries-dependent data) in relation to fishes available (fisheries-independent data). We found that hook-and-line fishing heavily targeted piscivorous fishes, net fishing caught the most diversity and the smallest fishes, and spear fishing targeted herbivores (particularly browsers and grazers) and piscivores.

In Chapter 4, we constructed a fisheries model to simulate population dynamics of reef fishes under gear-restricted management. We used the fisheries-dependent and -independent data from Chapter 3 to determine the catchability of functional group-size class combinations by each gear type (i.e., hook-and-line, net, and spear). Our baseline management scenario included all gear types with effort equally distributed among gears, and subsequent scenarios included a full-factorial design of gear restrictions. We found that permitting only spear fishing maintained the highest total biomass compared with other management scenarios, and prohibiting only spear fishing (i.e., permitting hook-and-line and net fishing) maintained the lowest total biomass across the full range of fishing effort tested. However, permitting only hook-and-line fishing generated the highest overall catch and maintained higher biomass of most functional groups compared with other management scenarios. These findings were primarily due to the low catchability of planktivores by spears and relatively high catchability by hook-and-line fishing, as planktivores were the most abundant functional group. Thus, gear-based management may be capable of achieving conservation and fisheries objectives simultaneously.

Chapter 5 is a recapitulation of results for Chapters 2, 3, and 4. Also, limitations and caveats of each chapter are discussed. The chapter concludes with a discussion on the future directions for this research.

Available for download on Sunday, August 14, 2022

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