Population dynamics and resilience of green abalone Haliotis fulgens in Isla Natividad

Abstract

Aim of my thesis project was to evaluate the efficacy of two marine reserves in Isla Natividad (Mexico) as a management tool for the green abalone (Haliotis fulgens) fishery. The first step was to develop and calibrate a demographic for green abalone to describe its dynamic when subjected to exploitation. To do so, I defined a size-based matrix model that allows incorporating information on growth, size-specific survival and size-specific fecundities; specifically, I conducted an extensive literary review to estimate the corresponding vital rates. I also used fishery data collected by fishermen on Isla Natividad to calibrate the model, i.e.to estimate density-dependence in the larval stage and size-dependent fishing mortality. In particular, through this model, I: - examined recent trends in abalone abundance in Isla Natividad and compared the current status of the stock with theoretical carrying capacity; - computed informative fishery reference points such as maximum sustainable yield (MSY) and effort that guarantees the MSY to compare them with current and historical fishery yields and exploitation rates; -assessed the recovery potential of abalone populations and investigate the likely trend of abundances and yields under different exploitation scenarios.

Then, I investigated if the reserves can increase, or at least maintain, the catch of abalones in the fishable areas through larval spillover, that is, a net export of larvae during the planktonic stage from the protected toward the unprotected zones. The expected increase in abalone abundance inside the marine reserves and the increase in mean size of sexual mature individuals, in fact, could positively influence the reproductive output of the population with an enhanced production of eggs, which, through larval dispersal, can contribute to the recruitment also in the areas outside the reserve. To explore the likely trends in abundance and catches of these commercially important species both in the short and in the long term, I extended the above-mentioned model formulation by defining a spatially-explicit version of the model. I assumed that the populations were distributed along a linear array of contiguous patches, representing the fishable and the protected blocks along the perimeter of the island. As abalones are sedentary species, I assumed that the connectivity between subpopulations was driven by the exchange of larvae in the planktonic phase. Dispersal kernels for larval movement were defined to describe movement probability between more or less distant patches. In particular, I: - examined the likely trends in abalone abundance and fishery yields catches under different assumptions of reserve establishment (intended as percentage of fishing grounds protected and size of individual no-take areas); - assessed if larval spillover effect can compensate for the decrease in abalone catches due to the reduction of the fishing grounds, and, if yes, after how much time are the benefits likely to occur; -analyzed if the results in terms of population trends and fishery benefits are sensitive to the spatial arrangement of the reserve.