Publication Date

2015-08-10

Availability

Embargoed

Embargo Period

2016-08-09

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine Biology and Fisheries (Marine)

Date of Defense

2015-05-01

First Committee Member

Nelson M. Ehrhardt

Second Committee Member

Jerald S. Ault

Third Committee Member

Andrew Bakun

Fourth Committee Member

Elizabeth A. Babcock

Fifth Committee Member

David J. Die

Sixth Committee Member

Christopher M. Legault

Abstract

The Indo-Pacific sailfish, Istiophorus platypterus, has significant ecological and socioeconomic importance in the tropical eastern Pacific Ocean (EPO). It is caught as bycatch in tuna commercial fisheries, but targeted in economically important catch-and-release recreational fisheries. The sustainability of this valuable resource had not been successfully assessed due to poor fishery landings and effort data with unreliable demographic estimates for sailfish. The goal of this dissertation was to assess lifetime demographics and stock mortality rates of sailfish and to infer ecosystem dynamics that drive availability of sailfish to recreational fisheries. This dissertation developed a new age and growth model for sailfish in the EPO. An improved set of growth parameters representing the regional distributional range of sailfish in the EPO were obtained by mixture distribution analyses on a large length frequency database and by incorporating size-at-age results from existing studies. A numerical algorithm, Statistical Age-Length Key (StALK), was developed using the new growth parameters and variance of sizes-at-ages to compute expected probabilities of ages-at-size. The StALK algorithm was validated with known simulated fishery age-structured data under varying levels of recruitment, exploitation, size selectivity, and random error. Simulation experiments showed the statistical robustness and limitations of StALK at producing reliable age frequency distributions at size from observed length-frequency distributions. StALK was applied to annual sailfish length-frequencies from 1991-2010 from the non-selective EPO purse seine fishery. Age frequencies resulting from StALK were used to estimate total mortality rates for a data-poor EPO sailfish fishery using catch curve analysis of resulting age frequencies and compared well with length-based methods. Age frequencies resulting from StALK for sailfish produced instantaneous fishing mortality rates that exceeded precautionary benchmarks (F0.1) for fishing mortality for all but two years. These results indicate that sailfish in the EPO are experiencing overfishing, indicating the need for urgent fishery management. Generalized multiple linear model analysis reveal that recruitment of EPO sailfish is dependent on time-delayed ocean current indices (e.g., intensity of northern equatorial currents). Ocean current indices significantly predict recruitment of age 5 sailfish, the first fully recruited age class into recreational fisheries. Sailfish CPUE in recreational fisheries appear to be driven by oceanographic variables such as sea surface height and ocean currents. These variables are responses to local upwelling and shallowing of the oxygen minimum layer which concentrates fish in limited areas, inflating sailfish catch rates in regional recreational fisheries. Satellite logbook systems installed on recreational vessels in Guatemala and Costa Rica demonstrated the negative effects of vessel crowding relative to the sailfish catch efficiency (fishing success). It is concluded that expansion of tourism-based recreational fisheries in the region could have deleterious effects on fishing success if these fisheries are not cooperatively managed throughout the Central American Isthmus.

Keywords

Population Dynamics; Sailfish; Eastern Pacific; Age and Growth; Recreational Fisheries; Mortality Estimation

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