Maps of anchovy length distributions in the Bay of Biscay at two seasons
|Author(s)||Petitgas Pierre1, Doray Mathieu1, Boyra Guillermo2, Duhamel Erwan1, Sobradillo Bea2, Martinez Urdane2|
|Affiliation(s)||1 : IFREMER
2 : AZTI
|Keyword(s)||Fish length distribution, Kriging, Anchovy, Bay of Biscay|
The anchovy (Engraulis encrasicolus) population in the Bay of Biscay is surveyed each year in May since 2000 and in September since 2003 by means of acoustic surveys. The integrated survey PELGAS (Doray et al., 2018) is run in May by France and covers the French shelf of the Bay of Biscay. Its objectives are to monitor the Bay of Biscay pelagic ecosystem in springtime and assess the biomass of its small pelagic fish species, including anchovy. Amongst many information on the ecosystem, the survey PELGAS provides knowledge on the adult anchovies during peak spawning. The survey JUVENA (Boyra et al., 2013) is run in September by Spain. It has a larger spatial coverage than PELGAS, including part of the Spanish coast and open ocean outside the shelf because it targets juvenile anchovy. It also provides knowledge on adult anchovy as well as other pelagic species. Both surveys are coordinated by the ICES Working Group on Acoustic and Egg Surveys for Small Pelagic Fish (WGACEGG), together with other pelagic surveys in ICES areas 7, 8 and 9. Survey protocoles are detailed in Doray et al. (2021). Briefly, fish backscatter data are recorded along the survey transect lines and pelagic trawl hauls are undertaken opportunistically to identify the echotraces to species and collect fish samples for biometric data. The trawl haul catches have provided the anchovy length data, from which the maps presented here are derived. At each trawl haul, the catch is sorted by species and weighted. A subsample by species is measured to estimate the species’ length distribution. The two maps presented here correspond to the average maps of anchovy length distributions in May and September, derived from the PELGAS and JUVENA trawl haul data series. The maps were obtained by kriging, following the procedure explained in Petitgas et al. (2011) for mapping functions instead of variables. The experimental length distribution at each haul was fitted by a linear combination of Legendre polynomials, the coefficients of which were co-kriged. The length histogram at each grid node was then deduced from the mapped coefficients. When the length distribution at a given haul was estimated with less than 40 individual fish, the haul was not taken into account for mapping. This threshold defined presence and absence of anchovy in the haul data sets. The trawl hauls from 2000 to 2019 were pooled for the PELGAS series (1965 stations) and from 2003 to 2020 for JUVENA (852 stations). The experimental length distributions were fitted with 19 Legendre polynomials for the JUVENA trawl haul data and 15 in the case of PELGAS. The mapping was performed on the same grid for both PELGAS and JUVENA, and with the same moving kriging neighbourhood. The grid has a mesh size of 0.25 x 0.25 decimal degree square. In addition to mapping the length distribution, presence/absence was mapped by ordinary kriging on the same grid and with the same neighbourhood for both PELGAS and JUVENA. The computations were performed in R (version 4.0.5) with the RGeostats package (version 13.0.1) freely available at http://rgeostats.free.fr. The map data files comprise the following information: the geographical coordinates of the grid points, the probability of anchovy presence and the probability of each length class of width 0.5 cm ranging from 2.5 to 21 cm.