Marine & Environmental Sciences Faculty Proceedings, Presentations, Speeches, Lectures

Making Lemonade from Lemons: Using Pelagic Longline Gear Behavior TDR Data for Insights Into Post-Hooking Behavior of Fishes

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61st Annual Tuna Conference, Lake Arrowhead, California, May 17-20, 2010

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Determining the actual fishing depths of pelagic longline gear has long been a goal of fisheries science, largely for the standardization of fishing effort through the use of habitat‐oriented modeling. Early efforts to record these depths used large temperature‐depth recorders (TDRs) along the mainline, but more recent work with improved (smaller) technology has resulted in the deployment of hundreds of microTDRs on the gangions themselves to determine actual hook depths. The use of baited hooks on these gangions resulted in the occasional catch of a fish on a gangion equipped with a microTDR. These records are therefore useless from a gear behavior modeling perspective and have been previously disregarded.

The collection of several hundred new and old microTDR records of caught pelagic fishes now allows some insight into actual post‐hooking fish behavior. A total of 490 records were examined from microTDR research by the senior author between 2003 and 2009, spanning 17 teleost and 13 elasmobranch species. Extracted data included time, temperature, and depth at hooking and death, and these were then matched with individual fish data, such as length. Analyses show a broad range of survival time on the hook, even within species. Hook location (internal versus external) and individual fish size were variable effects to the length of post‐hooking survival. For individuals that survived for multiple hours on the hook, three general patterns of movement were seen: surface association, constant vertical movement, and repeated vertical movement. Post‐hooking behavior patterns were consistent for some species (e.g., manta ray Manta sp.), but not others (e.g., swordfish Xiphias gladius).

Not surprisingly, the two most commonly used estimators of hook depths consistently overestimated the hooking depths for most species, suggesting that subsequent population analyses based on these estimated depths may be suspect. These data indicate that the present hook depth predictor equations may not provide sufficient information to extrapolate individual species’ habitat utilization and, by extension, a strong rationale for their use in pelagic longline fishing gear standardization efforts.

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