Marsac Francis, Floch Laurent. (2014). Skipjack tuna CPUE trends using alternative indices from the French purse seine logbooks. In :
16ème groupe de travail sur les thons tropicaux. Victoria : CTOI, 32 p. (Documents de Réunion ; IOTC-2014-WPTT16-41). GTTT : Groupe de Travail sur les Thons Tropicaux, 16., Bali (IDN), 2014/11/15-19.
Titre du document
Skipjack tuna CPUE trends using alternative indices from the French purse seine logbooks
Année de publication
2014
Type de document
Colloque
Auteurs
Marsac Francis, Floch Laurent
In
16ème groupe de travail sur les thons tropicaux
Source
Victoria : CTOI, 2014,
32 p. (Documents de Réunion ; IOTC-2014-WPTT16-41).
Colloque
GTTT : Groupe de Travail sur les Thons Tropicaux, 16., Bali (IDN), 2014/11/15-19
We used the French purse seine data to examine the trend of skipjack CPUEs for the period 1984-2013. Only sets made on drifting objects (logs and fish aggregating devices –FADs) are used in this analysis. Sensitivity tests were performed to evaluate the effect of the size of the core area in the north equatorial area (0-13N/45’70E) as well as effort minimal thresholds, on CPUE estimates (filtered nominal indices based on the catch per set). No significant effect was noted suggesting that the north equatorial area, as a whole, is a pertinent skipjack core area. A standardization procedure was applied to datasets based on CPUEs aggregated by 1°longitude/0.33° latitude and by month, to which corresponding environmental data such as the depth of the mixed layer, the sea surface anomaly, the speed of the current and the chlorophyll concentration were added. Generalized additive models (GAMs) were used to explore the shape of relationships with fishery-derived and environmental covariates, and undertake transformation in the dataset used by generalized linear models (GLMs). Seven models with various assumptions were tested. They denote a clear skipjack CPUE decline since 2002, with a steeper rate of decline since 2009. We suggest that the massive deployment of FADs during the 1990s and the development of efficient FAD fishing tactics have likely contributed to such decline, as more than 60% of skipjack is caught on FADs. However, environmental fluctuations are also at stake in such decline and notably the chlorophyll concentration which reflects the productivity level at the base of the food chain. The surface chlorophyll concentration has been anomalously low in the Somali Basin since 2006 and this could lead to detrimental foraging conditions for the upper trophic levels, or trigger spatial shift of the population in search of more productive areas. The biological and ecological processes associated to this decline would deserve more research. However, the fact that a sustained CPUE decline for skipjack has been occurring since 2002 could become a concern for the conservation of this stock.
Plan de classement
Pêches [040PECHE]
Descripteurs
PECHE THONIERE ; STOCK ; SENNEUR ; DISPOSITIF DE CONCENTRATION DES POISSONS ; EFFORT DE PECHE ; CAPTURE ; CHLOROPHYLLE ; VARIATION PLURIANNUELLE
