Abstract

This paper reports on a stock assessment of an assumed ‘southern Caribbean’ stock of king mackerel, based on length frequency data collected in Trinidad and Tobago mainly in the years 1996 to 1998 (total n=2200), and a second smaller data set from 2004 (n=558). Von Bertalanffy growth rates were first estimated using the combined 1996-98 data set in the FMSP LFDA package.  The analysis provided two sets of growth parameters, a ‘low L?’ model (L?=130cm, associated with a K of 0.35), and a ‘medium L?’ model (L?=155cm, with a lower K of 0.30), both of which provided equally good fits to the main modes in the data set. Total mortality rates (Z) were estimated for each of the two growth models as 1.26 and 1.70 for the 1996-98 data set, and as 1.99 and 2.50 for 2004.  With estimated natural mortality rates (M) of 0.59 and 0.51 for the low and medium L? growth models respectively, fishing mortality rates, F, were estimated as 0.67 and 1.19 for 1996-98 and 1.40 and 1.99 for 2004.  An ‘analytical’ or ‘per-recruit’ model was then fitted using the FMSP ‘Yield’ software, using a standard Beverton and Holt formulation.  Based on a draft objective of ”maintaining the sustainability of resources”, the F20%SPR reference point was adopted as an upper limit or threshold fishing mortality rate, that should not be exceeded.  The two growth models gave F20%SPR estimates of 0.80 and 0.66.  The 1996-98 fishing rates were thus either 16% below or 80% above the F20%SPR reference point, depending on which growth model was used.  For the higher 2004 estimates of fishing mortality rates, both models suggest that the fishery was operating well above the assumed ‘safe’ levels of the F20%SPR reference point (i.e. 85-202% above).  Consideration was given to adjustments in fishing effort levels that could be used to reduce F to below the limit reference points.  Guidance was also provided on closed seasons and fish size limits that could alternatively be used to maintain spawning stock biomass per recruit above the limit 20% level.  Due to the small length frequency sample size available in 2004 and the high level of uncertainty in the analysis, the study concluded that new length data should be collected urgently to clarify the current position of the fishery.  Responsible management will also require improved understanding of the stock distribution, and the harmonization of management measures in each of the states that share this stock

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