Abstract
Black scabbardfish, Aphanopus carbo, is a commercially important species that takes distant migrations throughout its life cycle. Sex steroids were measured by radioimmunoassay in the blood plasma of specimens caught off the Madeira Archipelago and mainland Portugal to link this species migratory path with its reproductive cycle. Furthermore, a pilot study using Mozambique tilapia (Oreochromis mossambicus) was designed to evaluate the effect of sample freshness on steroid levels because black scabbardfish blood was collected at separate times after specimens were caught. The changes in T and 11-KT concentrations between the time of blood extraction and the time after preservation did not statistically differ among the different methods applied. Therefore, measured black scabbardfish steroid concentrations were directly used in the subsequent data analyses. In females, E2 and in T concentrations peaked at a late stage of vitellogenesis. E2 concentration was significantly different between females caught off each area. Clustering E2 and T concentrations from all developing females resulted in the separation of two distinct groups, independently of their geographical area. In males, T and 11-KT were not significantly different between maturity stages. The hepatosomatic index of males caught off mainland Portugal was relatively high. This may reflect a mechanism for storing energy that will later be consumed during migration to the spawning grounds. The trend of sex steroids concentrations throughout the sexual maturation of the species is consistent with the morphological indicators and shows evidence of the reproductive and migratory pattern hypothesised for the black scabbardfish in NE Atlantic.
Highlights
The black scabbardfish, Aphanopus carbo Lowe, 1839, is an important commercial deep-sea species caught in Portuguese waters
In females caught off mainland Portugal, mean gonadosomatic index (GSI) significantly increased between stage 1 and stage 2 (W = 3544, p-value < 0.001)
Concerning males, mean GSI significantly increased from stage 1 to stage 2 (W = 488, p-value < 0.001) and from stage 2 to stage 3 (W = 845, p-value < 0.001) in specimens caught off mainland Portugal, and from stage 2 to stage 3 (W = 17,335, p-value < 0.001) and from stage 3 to stage 4 (W = 2494, p-value < 0.001) in specimens caught off Madeira, whereas the decrease from stage 4 to stage 5 was statistically significant (W = 8213, p-value < 0.001) in specimens from Madeira
Summary
The black scabbardfish, Aphanopus carbo Lowe, 1839, is an important commercial deep-sea species caught in Portuguese waters. This benthopelagic teleost fish is widely distributed along the NE Atlantic (Allain, Biseau, & Kergoat, 2003; Bordalo-Machado et al, 2009; Ehrich, 1983), where it undergoes large-scale migrations: the smallest specimens are reported further north and the largest specimens at the southernmost limit of distribution (Farias, Morales-Nin, Lorance, & Figueiredo, 2013; Ribeiro Santos, Minto, Connolly, & Rogan, 2013). A key question is why fishes off mainland Portugal do not develop beyond the pre-spawning stage, despite attaining sizes larger than the estimated length at first maturity (L50 = 102.8 cm) (Figueiredo et al, 2003)
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