Variation in the abundance of sandeels Ammodytes marinus off southeast Scotland: an evaluation of area-closure fisheries management and stock abundance assessment methods

In 2000, a sandeel fishery off SE Scotland, which commenced in the early 1990s, was closed in response to concerns that the fishery was having a deleterious effect on seabird breeding performance at colonies in the vicinity of the fishing grounds. Fishery-derived catch per unit effort (cpue) data are used together with three different fishery-independent survey techniques (acoustic, demersal trawl, and nocturnal grab survey) to assess variation in sandeel Ammodytes marinus population biomass in the area over the period 1997–2003, a period that included the last three years that the fishery was operating and the first four years of the sandeel fishing moratorium. Temporal trends in estimates of sandeel biomass derived from these different assessment methods were inconsistent and, on the basis of these alone, it was not possible to determine whether sandeel population biomass in the area had increased following the closure of the fishery. The different survey methods assess different components of the sandeel population; acoustic survey and fishery cpue quantified sandeels active in the water column, whilst demersal trawl survey quantified sandeels buried in the sediments. These data were collected at a time of year when sandeels were moving between the seabed sediments and the overlying water column. A grab survey also quantified sandeels buried in the sediment, but these data were collected at a time of year when the entire population should have been buried in the sediment. Differences between the different time-series were reconciled by taking account of the cumulative total primary production in each year prior to the surveys. On the basis of this, a model was developed that utilized acoustic and demersal trawl survey data to estimate the total sandeel population biomass. This model was validated using the nocturnal grab-survey data. The modelled data indicated that the biomass of sandeels 1+ years old increased sharply in the first year of the closure and remained higher in all four of the closure years than in any of the preceding three years, when the fishery was operating. The biomass of 0-group sandeels in three of the four closure years exceeded the biomass present in the three years of commercial fishing. Whereas the response of 1+ sandeels may have been a direct consequence of the closure, this is not likely to have been the case in respect of 0-group sandeels. The closure appears to have coincided with a period of enhanced recruit production.