Fishery Stability, Local Extinctions, and Shifts in Community Structure in Skates

Abstract

Skates are arguably the most vulnerable of exploited marine fishes. Their vulnerability is often assessed by examining fisheries catch trends, but these data are not generally recorded on a species basis except in France. Aggregated skate catch statistics tend to exhibit more stable trends than those of other elasmobranch fisheries. We tested whether such apparent stability in aggregated catch trends could mask population declines of individual species. We examined two time series of species-specific surveys of a relatively stable skate fishery in the northeast Atlantic. These surveys revealed the disappearance of two skate species, longnose skate ( Dipturus oxyrhinchus ) and white skate ( Rostroraja alba ) and confirmed a previously documented decline of the common skate ( D. batis ). Of the remaining five skate species, the three larger ones have declined, whereas two smaller species have increased in abundance. The increase in abundance and biomass of the smaller species has resulted in the stability of the aggregated catch trends. Because there is significant dietary overlap among species, we suggest the increase in abundance of the smaller species may be due to competitive release as the larger species declined. A consequence of this kind of stability is that declining species cannot be detected without species-specific data, especially in taxa exhibiting competitive interactions. This may explain why previously documented disappearances of two species of skates went unnoticed for so long. The conservation of skates and other elasmobranchs requires species-specific monitoring and special attention to larger species. Estabilidad de la Pesquería, Extinciones Locales y Cambios en la Estructura de Comunidades de RayasResumen: Las rayas son presumiblemente los peces explotados más vulnerables. Su vulnerabilidad es frecuentemente evaluada mediante el examen de tendencias de capturas pesqueras; sin embargo, con la excepción de Francia estos datos generalmente no son obtenidos a nivel de especie. Las estadísticas de las capturas agrupadas de rayas tienden a exhibir tendencias estables en comparación a otras pesquerias de elasmobranquios. Evaluamos si esta aparente estabilidad en las tendencias de las capturas agrupadas podría encubrir una disminución poblacional de las especies a nivel individual. Examinamos dos series de tiempo de estudios especie-específicos en una pesquería de rayas relativamente estable del Atlántico nororiental. Estos estudios revelaron la desaparición de dos especies de rayas, la raya nariz larga ( Dipturus oxyrhinchus ) y la raya blanca ( Rostroraja alba ) y confirmaron un declive previamente documentada de la raya común ( D. batis ). De las cinco especies restantes, las tres más grandes disminuyeron, mientras que las especies pequeñas incrementaron en abundancia. El incremento en abundacia y biomasa de las especies más pequeñas resultó en la estabilidad de las tendencias de las capturas agrupadas. Debido a que existe una sobreposición dietética entre especies, el incremento en abundancia de ...

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