Data availability for the evaluation of stock status of species without catch advice: Case study: turbot (Psetta maxima) and Brill (Scophthalmus rhombus)

Several commercially important demersal fish stocks for the North Sea fisheries are classified as “category 11” in the light of the EU policy paper on fisheries management (17 May 2010, COM(2010) 241). For stocks in this category, there is no STECF (Scientific, technical and economic committee for fisheries ) management advice, due to the unknown status of the stocks. The reason for this is that the data and information available to perform analytical stock assessments are highly uncertain or lacking. This document describes existing data and options for collecting new data for the evaluation of the state of category 11 stocks. We focus on turbot and brill in the North Sea. Existing data from logbooks and the market sampling program can be used to estimate LPUE series used in age-based stock assessment methods, similar to other commercially important flatfish species such as plaice and sole. Landings Per Unit of Effort (LPUE) data of the Dutch beam trawl fleet > 221 kW were standardised for engine power and corrected for targeting behaviour as described below and graphically shown in Figure 3.2.1. The methods are similar to those used to analyse commercial LPUE data for North Sea plaice, described in Quirijns and Poos (2010). Landing rates (LPUE) by market category were calculated for the period 2002-2010. The corrected LPUE series indicate an increase in commercial LPUE for both species during the period 2002-2006. For turbot, the LPUE stays stable in the first five years, increases between 2006 and 2008, and decreases afterwards. The increase in brill LPUE is larger, and occurs throughout the study period. Compared to other commercially important flatfish species relatively few brill and turbot market samples are taken. This reduces the ability to track the cohorts in the LPUE series of the older ages which is a prerequisite for reliable stock assessment estimates. Also, the time series currently spans only 9 years. Collection of additional data may therefore be desirable. Expanding the BTS survey will provide industry independent data and would therefore give easily interpretable results. The option of an industry survey is also a good option if the survey is already being executed for sole and plaice. However, both options are costly. Therefore, the option of increasing the samples of turbot and brill at the auction is relatively easy and relatively inexpensive and therefore at present the most promising. The methods used for this document will not per definition be applicable for all category 11 species. For dab, flounder, lemon sole and tub gurnard, samples at the auctions are taken to collect biological data. For these species it may be possible to raise the data in a similar way as was done in this report to estimate the age composition of the stock. For other species (witch flounder, horse mackerel, silver smelt, red mullet and squid) there is no market sampling. For these species, other methods will have to be developed.