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Integration of discards in the Roundnose Grenadier stock assessment in ICES Vb, VI, VII Convertir en PDF Version imprimable Suggérer par mail
09-12-2008. Lu 2698 fois.

Pawlowski L., Lorance P., Colloque Approche Systémique des Pêches, Novembre 2008, Boulogne-sur-Mer.
A paraître dans les actes du colloque (Aquatic Living Resources)

We present a series of exploratory stock assessments for roundnose grenadier (Coryphaenoides rupestris) from estimates of catch data based on information and assumption of discards and landing statistics. It is a common practice to consider landings data as a substitute of catch data to perform stock assessments. When a significant portion of the catch is discarded, this assumption is likely to be misleading as assessment methods will provide underestimated stock size.

Observations have shown roundnose grenadier discards account for about 30% of the catch in weight. Discards data for this species have been available scarcely through observer programs since 1997 to 2005 and exhibit relatively stable length distributions and ratios between discards and catch. Although landings data have been available since 1990, no information on discards was available before 1997. Length distributions from landings show the average pre-anal length has decreased from 20.7 cm in 1990 to 15.7 cm in 2007 resulting in a reduction of 50% of the mean individual weight (980g in 2007) and an increasing occurrence of overlapping class sizes between landings and discards in recent years. In order to rebuild the catch time series, it was necessary to make some assumptions regarding the missing information. In a first attempt, we assumed length distributions for discards had not changed over the 1990-2007 period. We applied an interpolated length distributions for discards and extrapolated the proportions of weights between discards and catch to all the missing years.

Exploratory stock assessments were carried through a series of separable virtual population analysis. Results show the hypothesis of stable length distributions is acceptable in recent years. However, for the early 1990s, catch length distributions were bimodal which is not realistic considering the slow growth and longevity of roundnose grenadier. These results imply the length distribution of discards has changed from the beginning of the fishery in 1990 to 1997 with probably bigger individuals discarded in the the past. These changes in distribution could reflect a pickier behaviour of the fishermen when bigger fishes were more abundant. With a decreasing individual size in the catches, some size classes that used to be discarded would now be kept for landings. Those changes could also be the consequence of the evolution of fishing depths over the years. Both hypotheses are explored through exploratory assessments based on informations from the observer programs, fishing efforts and vertical distributions of roundnose grenadier. 

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