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Incorporating Oceanographic Data in Stock Assessments of Blue Sharks and Other Species Incidentally Caught in the Hawai'i-based Longline FisheryProgress Reports (PDF): FY 2007, FY 2006, FY 2005, FY 2004, FY 2003, FY 2002, FY 2001
Blue sharks (Prionace glauca) comprise the bulk of the non-target species in the catch of pelagic longline fleets in the North Pacific. Historically, a large percentage of the blue sharks have been released alive but in the past decade there has been a dramatic increase in the number of sharks that are finned. Whether that has led to an equally dramatic increase in the number of blue sharks killed depends on the survival rate of blue sharks when released alive, which is not well known. Because of their life history characteristics of slow growth, late maturity and low fecundity, sharks in general are known to be particularly vulnerable to stock collapse as a result of over-fishing. However, there is considerable variation among shark species in their life history parameters and therefore in their productivity and robustness to fishing pressure. With appropriate management measures some shark species can be sustainably harvested. The purpose of this blue shark stock assessment is to determine the degree to which the blue shark population has been affected by fishing activity in the North Pacific and whether current fishing practices in the region need to be curtailed or adjusted in some way to ensure that the blue shark population is sustained.
One of the difficulties of doing stock assessments is determining the relationship between the nominal fishing effort and the actual, realized fishing pressure. With longline gear, effort is generally measured by the number of hooks deployed, the method (type of bait, lightsticks, time of day, soak time) and depth of hook deployment, and number of hooks set between floats. The number of hooks set between floats is the primary method used to adjust the depth of the hooks. This value has been used in stock assessments as part of a way to standardize longline effort by estimating the degree of overlap of the depth distribution of the hooks with the favored depth of range of the fish. The problem with this is that experiments with depth recorders on longlines have shown that hooks do not always reach their intended depth and that actual depth distribution of hooks is strongly affected by current shear. Therefore the depth distribution reckoned from the number of hooks between floats could often be in error, particularly in instances of high current shear.
The principal objective of this project is to develop an improved stock assessment technique that refines the above aproach to standardizing effort by considering oceanographic conditions affecting depth distribution of longline hooks, in particular the occurence of shear currents. This will require input of oceanographic data of appropriate levels of resolution in space and time and with a geographic scope and historical depth equivalent to the catch and effort data available for input to the assessment. Such data is to be the output of another PFRP project, Development of oceanographic atlases for pelagic and insular fisheries and resource management of the Pacific Basin (PIs: R. Brainard, D. Foley and J. Sibert). Until such data from this project becomes available project investigators will conduct proof-of-concept exercises, making use of available oceanographic data as well as simulated data. The latter will involve developing a model to produce simulated fishery data based in part on simulated oceanographic data.
Once real oceanographic data are available, an index of shear current strength will be included as an effort standardization factor or as an input to a catchability sub-model of an assessment model. The second stage of the project will feature a horizontally disaggregated model that would necessitate inclusion of fish movement as well as the input of spatially disaggregated catch and effort. Project investigators envision incorporating some index of oceanographic frontal development in a sub-model governing the rate of fish movement from area to another. Project investigators will try various assessment models from simple production models to much more complex models, such as the age and spatially structured MULTIFAN-CL.
Project investigators hope to research other environmental features, such as oceanographic fronts which can affect the seasonal movements and aggregations of pelagic fishes. This would subsequently affect the degree of overlap of horizontal distributions of the fish and longline effort.
1 funding for this project awarded in August 2001.
Dr. Pierre Kleiber
National Marine Fisheries Service
2570 Dole Street
Honolulu, Hawaii 96822
Phone (808) 983-5399
FAX (808) 983-2902
Dr. Hideki Nakano
National Research Institute for Far Seas Fisheries
5-7-1 Orido, Shimizu 424
This page updated July 13, 2010