Long-term time series of mussel and scallop abundances in the Bay of Seine (Normandy, France).

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 (> 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’.在法国诺曼底地区，尤其是塞纳湾（Bay of Seine），大扇贝（Pecten maximus）与紫贻贝（Mytilus edulis）是当地捕捞船队主要采收的双壳类软体动物物种。该区域的扇贝种群是欧洲最重要的种群之一，且近数年呈现增长态势。其捕捞产业目前支撑着一项重要的经济活动——年上岸量达15000吨。然而，扇贝种群增长并非单调稳定，幼扇贝的年度补充量波动幅度极大。 对于紫贻贝而言，由近40艘渔船组成的船队开发了潮下带的天然贝床，年上岸量介于1000至5000吨之间；受幼贻贝年度附着情况波动影响，极端情况下年上岸量可达30000吨。但自2014年起，年度前瞻性监测调查显示整个区域的幼体补充量出现了大幅下降。 双壳类附着的长期数据序列通常呈现出极强的波动模式，这源于软体动物补充过程对环境的高度敏感性。尽管历史上扇贝与贻贝种群的丰度曾出现剧烈波动，但仍需通过年度监测项目估算年度种群丰度，并依据现有资源规模调整捕捞努力量。 应利益相关方与渔民的要求，法国海洋开发研究院（Ifremer, French Research Institute for Exploitation of the Sea）与区域渔业委员会自1992年起开展扇贝监测调查，自1982年起开展贻贝监测调查。 本数据集整合了上述调查的全部结果，以提升此类数据的复用潜力。该数据库可用于量化环境变化对两种海洋双壳类物种幼体补充的影响，并开展气候变化情景下的幼体补充预测模拟。 针对扇贝，每年在塞纳湾南部（大致位于北纬49°35′以南区域）的若干1平方海里采样单元中开展采样。采样单元的数量依据前一年度调查的结果确定，以确保丰度估算值的代表性。在每个采样单元中，捕捞渔船使用2台孔径宽2米的贝类拖网开展作业；采收的个体按年龄组分类并计数，后续研究人员计算得到各年龄组、各拖网覆盖表面积的丰度指数。 针对紫贻贝，每年均对塞纳湾西南部的5处小型贝床开展调查。各贝床的位置与范围如下： 1. 格朗康（Grandcamp）贝床：大致介于西经1°07′至0°56′、北纬49°26′至49°23′之间； 2. 拉瓦诺维尔（Ravenoville）贝床：边界范围为西经1°16′至1°07′、北纬49°32′至49°25′； 3. 勒维尔（Réville）贝床：边界范围为西经1°15′至1°10′、北纬49°37′至49°34′； 4. 穆拉尔（Moulard）贝床：边界范围为西经1°15′至1°10′、北纬49°41′至49°38′； 5. 巴夫勒尔（Barfleur）贝床：边界范围为西经1°21′至1°10′、北纬49°48′至49°40′。 在年度调查期间，租用的渔船按照适配贻贝分布的采样设计，通过拖网作业勘查各贝床，采样区域被划分为若干1平方海里的单元。每年针对每个贝床，调查人员在贝床外围的采样单元中完成1次拖网作业，以确定贻贝的覆盖范围。待贝床边界确定后，将采样强度均匀分布在该区域内。 在每一次拖网作业中，调查人员对采收的贻贝进行称重，并从收获个体中抽取子样本开展个体计数、体长测量与单独称重。后续研究人员估算得到各体长组的平均个体质量，并推导得到各采样单元内各体长组的贻贝数量。此处的丰度指数以拖网捕捞产量指数表示，单位为每分钟单台拖网的捕捞千克数。由于小型贻贝的可捕捞性较低，该指数仅针对商业规格（体长＞40mm）的贻贝计算。 本数据集共计包含186条记录与4个字段，字段名称依次为"year"、"species"、"area"与"index"。