It has been hypothesized that the geographical location of scallop beds in extensive shelf regions mirrors hydrographic structures (e.g. frontal systems) that favor the retention/concentration of pelagic larvae. Large, discontinuous concentrations of the Patagonian scallop (Zygochlamys patagonica) are known to have occurred recurrently (for more than 30 yr) at certain geographical locations over the extensive Patagonian shelf. These stocks, exploited since 1996, currently support one of the most important scallop fisheries in the world. Here, we investigate whether those aggregations are spatially coincidental with major frontal systems. Several pieces of information were used: historical survey data documenting the geographic distribution of the Patagonian scallop beds, catch and effort data from the commercial fleet, oceanographic data on frontal systems, and remote sensing imagery. We found that large-scale aggregations do match the location of three major and very different frontal systems in the southwestern Atlantic: the Shelf-Break Frontal System, the Northern Patagonia Frontal System, and the Southern Patagonia Frontal System. We describe the three frontal systems and their associated scallops fishing grounds and discuss which processes can contribute to sustaining the productivity of the scallop grounds in each case.
Male fiddler crabs compete for access to mates mainly by displaying an enlarged claw. One cost of this large claw is presumed to be higher exposure to predators. The fiddler crab Uca uruguayensis is used as a food source by several Neotropical migratory shorebirds. We investigated whether crab density and male claw-waving behaviour increase predation by shorebirds. The sex ratio of crabs on the surface was strongly male biased. Male crabs showed a daily succession of feedingwaving-feeding activities and their dry mass at the site decreased with a high density of crabs, suggesting stronger malemale competition than at sites with a low density of crabs. Sex-specific predation varied according to shorebird species. The Ruddy Turnstone (Arenaria interpres) preyed intensively on male crabs when they were courting and at sites with a high density of crabs, the Black-bellied Plover (Pluvialis squatarola) and the Lesser Golden Plover (Pluvialis dominica) preyed on crabs of both sexes, and the Whimbrel (Numenius phaeopus) foraged intensively on females. When estimating the net effect of all predators, we found that the four shorebird species together were able to consume between 0.1 and 0.2% of the crab population, and consumption did not differ between the sexes of crabs. These mortality rates indicate a low source of mortality, likely unable to produce deviations in the sex ratio of the population. Thus, our results indicate that a trait thought to be sexually selected does not necessarily lead to a disproportionate increase in mortality due to predation.Résumé : Les crabes violonistes mâles font compétition pour les femelles principalement en exhibant leur pince élargie. Un des coûts associés à cette grande pince est sans doute une exposition accrue aux prédateurs. Le crabe violoniste Uca uruguayensis sert de nourriture à de nombreux oiseaux de rivage migrateurs néotropicaux. Nous avons vérifié si la densité des crabes et le comportement d'agitation de la pince par les mâles augmentent la prédation par les oiseaux de rivage. Le rapport mâles : femelles des crabes à la surface favorise nettement les mâles. Les crabes mâles ont une succession journalière d'activités d'alimentation, d'agitation de la pince, d'alimentation et ils ont une masse sèche plus faible au site où la densité des crabes est la plus grande, ce qui fait croire à un niveau de compétition entre les mâles plus élevé qu'à des sites à faibles densités de crabes. La prédation spécifique au sexe varie selon les espèces d'oiseaux. Le tourne-pierre à collier (Arenaria interpres) fait une forte prédation des crabes mâles pendant leur comportement de cour et aux sites de fortes densités de crabes, le pluvier argenté (Pluvialis squatarola) et le pluvier doré d'Amérique (Pluvialis dominica) se nourrissent de crabes des deux sexes et le courlis corlieu (Numenius phaeopus) fait une chasse intense des femelles. En estimant l'effet net de tous les prédateurs, nous avons découvert que l'ensemble des oiseaux de rivage arrive à consommer entre 0,1 et 0,2 % de la populati...
A large part of fishing behavior is choosing where to fish. Trawl skippers usually choose between known fishing opportunities, which are observed as groups of trawls that are conducted in the same portion of a fishing ground, or go exploratory fishing. We outline a simple clustering method based on Euclidean distances between trawls that offers a more realistic way of defining fishing opportunities than grid cells or statistical areas. The resulting cluster tree of trawls is divided into individual groups of trawls (fishing opportunities) using a recommended cut point. Our method correctly classified simulated trawls into fishing opportunities. Fishing opportunities were obtained for vessels in the British Columbia groundfish trawl fishery; each vessel usually fished at a wide variety (mean 26, standard deviation 16, range 269) of fishing opportunities. Within each fishing opportunity, trawls generally caught similar species. In the Argentina scallop fishery, our method was able to divide exploratory from regular fishing trawls, with obvious applications for catch-per-unit-effort calculations. Our method could also be used to detect positional errors in data from these fisheries. Fishing opportunities could provide indications of how fishermen might react to marine protected areas and to the imposition of quotas on multispecies fisheries.
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