Assessment of Hong Kong’s Inshore Fishery Resources

T. J. Pitcher, R. Watson, A. Courtney, D. Pauly, (1998). “Assessment of Hong Kong’s Inshore Fishery Resources.,” Fisheries Centre Research Reports No. 6 (Fisheries Centre, University of British Columbia, Vancouver, B.C., Canada.

This work aims to determine the exploitation status of Hong Kong’s inshore fishery resources, and the likely impact of management measures on the coastal ecosystem. This report describes results of the assessment work performed by the Fisheries Centre, UBC, between April 1996 and December 1997. Biomass and catch have been estimated by season and sector with survey data: from regular monthly samples of benthic resources using a prawn trawl, from samples of pelagic resources using a purse seine, and from catch estimated from an interview survey of fishers. Benthic biomass is also estimated in 18 spatial sampling strata. Prawns are included but the work does not cover shellfish. Total inshore resource biomass is estimated as about 9000 tonnes annual average ( = 4.9 tonnes km-2), of which 85% is comprised of pelagic species. There is a strong seasonal pattern, seen most strongly in the pelagic species, but also present in benthic resources, with total biomass peaking at over 27,000 tonnes ( = 15 tonnes km-2) in August and dipping to 1700 tonnes ( = 0.9 tonnes km-2) in February. These results are subject to uncertainties in estimating swept areas and in extrapolating from more detailed work on individual species. Based on estimating the probability distribution of the catch rates of individual vessels in seven gear sectors and by species, the total catch in Hong Kong waters is estimated as 14,700 tonnes (7.8 tonnes km-2). A detailed breakdown of this catch by gear sector and species is provided. Very wide confidence limits reflect a high variance in individual vessel catch rates, and the results are subject to considerable uncertainty deriving from the adoption of an interview protocol to estimate catch. Detailed assessments of exploitation status have been carried out for 17 individual species, four of which are crustaceans. Growth parameters and mortality parameters have been fitted to survey data, and age data derived from otolith readings. Growth is fitted by least squares techniques, length frequency analysis, and estimates take account of many published values from the literature. Mortality includes total mortality, estimated largely by cohort slicing, offshore migration with age for certain demersal species, estimated by a novel method, and present fishing mortality calculated by two alternative methods. Yield- and biomass-perrecruit analyses have been employed to evaluate exploitation status. Uncertainties have been explicitly defined and addressed through confidence limits placed on most estimates using Monte Carlo simulation techniques. Twelve of the 17 species are heavily overexploited, while the remainder, principally the small high-turnover species, fall into the fully exploited category. Very approximate sustainable yield estimations based on calculating unexploited biomass suggest that catches of larger and slower-growing species might be roughly doubled with optimal management. A major uncertainty is the equilibrium assumption made by all of these methods, that recruitment will not be greatly affected by increases in abundance. Multi-species bioeconomic analysis for the trawl fishery, based on parameters from the individual species, conflates the assessment optima for the individual species in terms of their relative value. The results suggest that long term yield for the 17 species might be roughly doubled by increasing mesh size, but are sensitive to assumptions made in estimating relative recruitment factors among the species. A trophic mass-balance model of the Hong Kong inshore ecosystem is constructed from information derived from the analysis and from the literature. The model includes shellfish, marine mammals, and all living components of the system. The model is used to predict the impact of six scenarios of changes in management on the relative abundance of sectors of the resource. Halving the current fishing mortality results in considerable benefits for all fishery sectors, and those with a conservation focus such as marine mammals. The full benefits of such a policy may, however, take a decade to be realised.