The influence of seasonal upwelling on the spatial and vertical distribution of sailfish (Istiophorus platypterus) in the Eastern Pacific Ocean

Samuel J Friederichs, Purdue University

Abstract

Overfishing has become an increasing threat to large pelagic fish in all oceans. Immediate actions to reverse this trend, such as decreased fishing effort and high-seas protected areas, are necessary to halt further declines. In order to implement effective conservation measures there must be a greater understanding of how pelagic fish relate to features in their environment. Certain features, such as upwellings, are known to attract these predators however; they are often difficult to predict and manage. Despite this difficulty, there are areas, such as the Eastern Pacific Ocean off Central America, that provide predictable seasonal upwellings that attract large pelagic fish. An upwelling in this region, known as the Costa Rica Dome, follows an annual progression characterized by well formed coastal currents during the summer followed by strong upwelling and mesoscale eddy formation in the winter. Many pelagic predators are known to associate with this upwelling, including the Pacific sailfish (Istiophorus platypterus). In this study, nine sailfish were equipped with pop-up archival transmitters between June 2008 and June 2009. The fish were captured from sportfishing vessels off northwestern Costa Rica. The transmitters were programmed to transmit in real-time when the fish surfaced allowing for accurate location data as well as depth and temperature data downloads. Location data were overlaid on satellite composite imagery of geostrophic velocity and direction, sea-surface temperature, chlorophyll-a concentrations and bathymetry. V-variant Rayleigh tests were used to compare mean sailfish movement vectors and current vectors to determine correspondence. Corresponding vectors were then ranked according to the increasing value of environmental variables (current magnitude, SST, and chlorophyll) to examine the relationship between current correspondence and environmental changes. Dive data were used to examine depth and temperature preferences as well as habitat extremes. From this data, both high-use and maximum habitat usage were calculated based on the depth of the water column. Seven of the nine tags provided a total of 205 monitoring days with a mean displacement of 581 km from point of capture and a tendency toward northward movement. The amount of similarity between fish movement and current varied by individual however; five fish were shown to associate with local currents at varying temporal scales. Association to currents with respect to environmental variables showed three distinct relationships: near-logistic fit; variable fit; and no association. Variable fit was the most consistent relationship observed primarily during non-upwelling conditions while a near-logistic relationship was only observed during upwelling conditions. Dive data were consistent across all tags showing that sailfish utilize a small percentage of the water column (1%) in a narrow thermal range (<2 >C°). Deeper dives did occur, but they appeared to be related to the location of 18 C° isotherm within the water column. Given this information, the annual cycle of the Costa Rica Dome upwelling was shown to affect the spatial distribution of sailfish, which displayed a tendency to move toward regions of upwelling. Despite an effect on spatial distribution, it appears that upwelling conditions do not influence the vertical distribution of sailfish. The narrow habitat preference seen across all individuals, regardless of season, suggests that vertical distribution is more closely associated with temperature. The presence of a predictable seasonal upwelling, such as the Costa Rica Dome, and these influences on sailfish distributions suggest an important spawning habitat which presents a unique conservation opportunity to effectively manage sailfish stocks within the Eastern Pacific Ocean.

Degree

M.S.

Advisors

Paladino, Purdue University.

Subject Area

Biological oceanography

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