Julie E. Brown

Document Type

Internship Report

Publication Date

Fall 2012


As apex predators, billfish occupy an important position within the context of Ecosystem Based Fisheries Management (EBFM) for the pelagic environment. EBFM seeks to protect entire systems to restore or preserve productivity, which is historically defined as a yield of consumable products. This definition, however, should be expanded to include other services to humans, such as the economic revenues generated by sport fishing. The social services provided by billfishing are directly impaired by the actions of the commercial fishing industry, which causes substantial mortality to billfish populations as bycatch in industries targeting much higher commercially valued species like tuna. Because bycatch species are not valuable for consumption, their conservation has been mostly ignored by international management institutions responsible for maintaining sustainable fish populations. The exception is the swordfish, which has a high value in seafood markets, and therefore its conservation has received priority. These institutions traditionally select biological reference points like Fmsy (the level of fishing mortality which achieves MSY) which maximize the commercial yield, and have had varied success in limiting the fishing effort which has, in some cases, reduced populations of billfish to dangerously low abundances. The low abundances witnessed in the billfish stocks diminishes opportunities for sport fishing to occur, and ultimately reduces the diversity and resilience of the ecosystem, and the ecosystem services provided. More conservative targets for fishing have been proposed to protect stocks from overfishing due to environmental and human uncertainties. For three species of billfish (white marlin, striped marlin, and swordfish) I calculated one of these reference points, F0.1 using biological parameters from primary literature. The model also incorporated aspects of the fishery such as age at entry. The calculation revealed several sources for uncertainty in the biological growth of the individuals, and we were forced to make assumptions to more accurately model the population dynamics for each species. The resulting fishing mortality targets for white marlin, striped marlin, and swordfish are 0.24, 0.44, and 0.33 respectively. I also compared these reference points with those calculated by various management institutions, and the actual current fishing mortality. Against the backdrop of historical management decisions for each billfishery, certain actions could be taken to further protect against overfishing for these important species. Firstly, more accurate growth parameters are needed to properly asses the billfish and calculate targets for sustainable exploitation. Secondly, current regulations could be expanded to reduce the effort in various ways such as temporal and spatial bans on longlining, gear restrictions, and bycatch quotas. Management must use a precautionary approach to maximize the ecosystem capital in the pelagic environment by protecting the sustainability of both target and non-target species such as the billfish, which comprise an integral component of healthy ecosystems and productive human industries.


Department: MBF

MPS Track: Fisheries Science

Location: University of Miami, Central American Billfish Association

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