DOI: 10.5281/zenodo.5180683
title: Long-term time series of mussel and scallop abundances in the Bay of Seine (Normandy, France).

description: In Normandy (France), and particularly in the Bay of Seine, the great scallop Pecten maximus and the blue mussel Mytilus edulis are the main bivalve molluscs species harvested by the local fishing fleet. In this area, the scallop stock is one of the most important in Europe and appears to be growing since a few years. Its exploitation actually sustains an important economic activity (with a landed volume of 15.000 tons per year). However, population growth is not monotonous and yearly recruitment of young scallops seems to be very variable. For mussels, a fleet of nearly 40 boats exploited the natural banks in the subtidal area, with a landed volume ranging between 1.000 and 5.000 tons, exceptionally reaching 30.000 tons per year depending of the yearly fluctuations of the juvenile mussels’ settlement. Since 2014, however, the yearly prospective campaigns have pointed a dramatic decline of the recruitment throughout the area. Long-term data series on bivalve settlement usually show strongly fluctuating patterns, due to the environmental sensitivity of mollusk-recruitment process. While scallop and mussel stocks historically presented strong fluctuations of abundance, yearly monitoring programs were required to estimate annual abundance, and to adjust the fishing effort in line with the available resources. In accordance with stakeholders and fishermen, Ifremer (the French Research Institute for Exploitation of the Sea) and the Regional Fisheries Committee conducted these campaigns since 1992 for scallops and 1982 for mussels. Here, we present the dataset synthetizing the results of these campaigns in order to extend the reuse potential of these data. Particularly, database may help to quantify the effects of environmental variations on recruitment of two marine bivalve species and forecast simulations of recruitment under climate change scenario For scallops, a various number of one nautical mile squares were sampled each year in the southern part of the Bay of Seine (approximately south of 49°35 north latitude). This number was determined following the results of the campaigns conducted in the previous years, to ensure the representativeness of the abundance estimators. In each sampling unit, the fisher boat hauled 2 survey dredges with a 2 meters wide aperture. Harvested individuals were then classified by age-class, and numbered. Later, we calculated back an abundance index per age-class and per surface unit covered by the dredges. For mussels, 5 small banks located in the south-western part of the Bay of Seine were prospected each year. Grandcamp bank is approximately located between 1°07’ W and 0°56’ W, and 49°26’ N. and 49°23’ N. Ravenoville bank boundaries are 1°16’ W-1°07’ W, and 49°32’ N-49°25’N. Réville bank boundaries are 1°15’ W and 1°10’ W, and 49°37’ N and 49°34’ N. Moulard bank boundaries are 1°15’ W and 1°10’ W, and 49°41’ N and 49°38’ N. Barfleur bank boundaries are 1°21’ W and 1°10’ W, and 49°48’ N and 49°40’ N. During the annual campaigns, chartered fishermen vessels prospected the different banks by dredging, following a sampling design adapted to the mussel distribution and divided by a various number of one mile squares. Each year and for each bank, they realized one haul in the sampling squares located in the periphery of the bank to determine the mussel coverage. Once the boundaries of the bank delimited, the sampling effort was uniformly distributed within the area. In any one haul, the mussels were weighted and a subsample of the harvested individuals were numbered, measured and weighted individually. Later, we estimated the mean individual mass per size-class, and deduced the number of mussel per size-class and per sampling unit. In this case, the abundance index consists in an index of dredging-yield, expressed in kilograms per minute of hauling one dredge. This index was calculated for the commercial-sized mussels (&gt; 40 mm) only, because of the reduced catchability of smaller mussels. The dataset contains 4 columns and 186 rows. The columns are ‘year’, ‘species’, ‘area’, ‘index’.