Title: The use of surplus production models and length frequency data in stock assessments: explorations using Greenland halibut observations

Type:
Final project
Year of publication:
2007
Publisher:
UNU-FTP
Place of publication:
Reykjavík
Number of pages:
51
Supervisors: Einar Hjorleifsson

Abstract

Production models and length-frequency based methods can be used in situations where data are limited. The West Nordic stock of Greenland halibut (where limited data are available) was assessed using three different fittings (equilibrium, linear and nonlinear) with a surplus production model and length based methods for estimating Z. The data analysed were: CPUE indices from (1) the Icelandic bottom trawl fleet 1985-2006, (2) the Icelandic fall survey 1996 – 2006, and (3) the Greenlandic fall survey 1998 – 2006 (2001 missing), along with total landings from the fishery and length frequency data sampled from the commercial fishery. The three fittings of the surplus production model produced r„s (~0.4) and q„s(~0.005) that were similar but K„s and MSY„s that were different. The regression method returned the highest MSY of 44,459 t and the nonlinear fitting the least of 28,000 – 30,000 t estimated by the Excel and ASPIC model. Indications are that total fishing mortality is increasing while average biomass, surplus production and total yield is decreasing. The relative biomass (B/BMSY) trend is predicted to have steadily declined. Using length at age data the growth parameters for the species were estimated to be L∞ = 100, K = 0.937 and t0 = -1.347, and mortality estimates from the length frequency (0.1 – 0.2) are lower than those predicted by the surplus production model (0.3 – 0.4). In terms of the overall outlook for the fishery; at the current rate of fishing mortality, catches will decline since the biomass is predicted to decline and there is a high probability of stock collapse if this trend continues. All indications are that fishing mortality needs to be reduced. Based on the results of forward projection, if fishing mortality is reduced by approximately 60% of the present fishing mortality. This translates to a catch of 8100 tonnes in 2007, with increases in catches in consecutive years as the stock starts to rebound.

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