From coexistence to competitive exclusion: can overfishing change the outcome of competition in skates (Chondrichthyes, Rajidae)?

Abstract

ABSTRACT.Competition for food could be a major force driving changes in the community structure of skates (Rajidae) subjected to fishing exploitation. Under this hypothesis, small skates are released from competition with larger skates after fishing has depleted the larger species. Here, we compare the abundance patterns of two sympatric skates with similar niches but different life histories, Bathyraja albomaculata (larger and slow-reproducing) and Bathyraja macloviana (smaller and faster-reproducing), before (1971, 1978) and after (1998)(1999)(2000)(2001)(2002)(2003)(2004) a 108% increase in industrial bottom trawling on the southeastern South American shelf in order to test the prediction that B. macloviana should competitively exclude B. albomaculata after the increase in fishing mortality. In 1971 and 1978, there was no relationship between the abundance of both species, indicating that they coexisted over large scales. In 1998scales. In -2004, the relationship between the abundances of these skates was bell-shaped, indicating that both species increased in abundance at low densities until peaking, after which B. albomaculata decreased when B. macloviana became more abundant, consistent with resource competition. We tested whether food may be a potential limiting resource by comparing the diet of both species. The two species consumed mostly polychaetes, differing only in the consumption of polychaetes from the family Nephthyidae, which was much higher for B. macloviana. Bathyraja macloviana could replace B. albomaculata at high densities when food resources may become scarce. These results support the hypothesis that competition release is an important factor explaining the changes in skate communities in overexploited areas. Keywords: Bathyraja albomaculata, Bathyraja macloviana, Rajidae, diet overlap, resource competition, elasmobranch fisheries, Argentina.De la coexistencia a la exclusión competitiva: ¿Puede la sobrepesca cambiar el resultado de la competencia en rayas (Chondrichthyes, Rajidae)?RESUMEN. La competencia por el alimento podría ser una fuerza importante detrás de los cambios en la estructura de las comunidades de rayas (Rajidae) bajo explotación pesquera. Según esta hipótesis, las rayas pequeñas son liberadas de la competencia por las rayas de mayor tamaño, al disminuir la abundancia de éstas últimas por la pesca. En este trabajo, se comparan los patrones de abundancia de dos rayas simpátricas con nichos similares pero con diferentes historias de vida, Bathyraja albomaculata (mayor y de reproducción lenta) y Bathyraja macloviana (más pequeña y de reproducción más rápida), antes (1971, 1978) y después (1998)(1999)(2000)(2001)(2002)(2003)(2004) de un incremento del 108% en el arrastre de fondo industrial en la plataforma sudeste de América del Sur, para evaluar la predicción que B. macloviana excluiría competitivamente a B. albomaculata después del aumento en la mortalidad por pesca. En 1971 y 1978, no hubo relación entre las abundancias de ambas especies, indicando que, a escalas grandes,...

Interannual variability in the skate assemblage on the South Patagonian shelf and slope

“…Bathyraja albomaculata is the smallest of the four predominant Falkland species but can still be susceptible to fishing pressure. Ruocco et al () found that on the south‐eastern Patagonian shelf, abundance of B. albomaculata decreased since development of a trawl fishery, while abundance of a smaller skate Bathyraja macloviana (Norman 1937) increased. Ruocco et al () concluded that different populations of B. albomaculata could be present in the south‐west Atlantic Ocean, in which case the Falkland Islands might have a local population that is not buffered over a broader spatial range.…”

“…Significant abundance increases of some species but not others may nevertheless signal changes in the composition of the assemblage under continuing exploitation. In the south‐west Atlantic (Ruocco et al , ) and north‐east Atlantic (Walker & Heessen, ; Walker & Hislop, ; Dulvy et al , ), changes in skate species composition have shown increasing abundances of small species as large species decreased. In the north‐west Atlantic, changes have been less clearly related to size (Stevens et al , ; Swain et al , ), although life‐history analyses have also indicated that larger species are more vulnerable to decline (Frisk et al , ).…”

Effects of intrinsic and extrinsic factors on the diet of Bathyraja macloviana, a benthophagous skate

“…Bathyraja macloviana is a benthic predator that feeds mainly on polychaetes followed by amphipods, isopods and decapods. This results agree with published information on diet composition of this species in the same region (Mabragaña et al ., 2005;Scenna et al ., 2006;Ruocco et al ., 2012). Many species of elasmobranchs such as B. macloviana are not top predators, and have received little attention, with the potential effects of these predators dismissed in ecological systems (Vaudo & Heithaus, 2011).…”

“…Females mature between 520 and 540 mm L T , whereas males mature between 519 and 535 mm L T (Paesch & Oddone, 2009). Bathyraja macloviana is a benthophagous fish that feeds mainly on polychaete worms and a small proportion of amphipods, isopods and decapods (Mabragaña et al ., 2005;Scenna et al ., 2006;Ruocco et al ., 2012). Although these studies provide important data on the diet composition of B. macloviana, no information about the interactions among intrinsic and extrinsic factors (e.g.…”

Species biology of elasmobranch by-catch in bottom-trawl fishery on the northern Patagonian shelf, Argentina

“…reducing predation) or indirectly (i.e. reducing competition) af fect other species of the marine community (Stevens et al 2000, Shepherd & Myers 2005, Ruocco et al 2012, since they are key species in most ecosystems (Stevens et al 2000). Despite the key role that elasmobranchs play in ecosystems, the basic biological information needed for responsible management is lacking for many species, including minimum, maximum and average sizes, age-at-maturity estimations and lengthto-weight relationships.…”