Influence of sampling gear in assessment of reproductive parameters for bluefin tuna in the western Mediterranean

Abstract

Comparative histological and stereological analysis of gonads from longline-and purse seine-caught Atlantic bluefin tuna Thunnus thynnus was undertaken to assess the reproductive potential of the population in the western Mediterranean breeding area. Bluefin tuna sampled by purse seine clearly had higher gonadosomatic index values (GSI) than those caught by longline. While no remarkable histological differences were found between purse seine-and longline-caught males, the ovarian histology indicated a higher level of maturity in females from purse-seine samples, where most specimens were found to be in active spawning (AS). Within the longline sample the proportion of active spawning females was considerably higher at the end of the fishing season (midJune to mid-July) than earlier in May to mid-June (52.8 vs. 24%). Stereological analyses revealed that the samples collected by the 2 sampling gears differed primarily in the numbers of migratorynucleus oocytes, which were overwhelmingly more abundant in purse-seine catches, with average relative fecundity: 59 oocytes g -1 vs. 1.2 oocytes g -1 (purse-seine vs. longline). The frequency of ovaries with postovulatory follicles (POFs) in purse seine-caught bluefin tuna was 83.7%, equivalent to a spawning interval of 1.2 d; in contrast, only 32.6% of the females in longline catches had POFs, representing a spawning interval of 3.1 d. Assuming an average individual duration of the spawning process of 14 d, and considering the estimates of fecundity and spawning interval obtained from purse-seine samples, the average annual fecundity of bluefin tuna in the Balearic spawning ground would be approximately 77 × 10 6 eggs (relative fecundity of 702 eggs g -1 BW [body weight]).

Cannibalism among size classes of larvae may be a substantial mortality component in tuna

“…), which is the average individual duration of the spawning process in an adult BFT female (Medina et al 2007). Length at hatch was set at 3.5 mm, when the yolk sac is used up, and input densities ranged from 0.01 to 1.2 larvae m -3…”

Geographically and environmentally driven spawning distributions of tuna species in the western Mediterranean Sea

“…The 3 species are iteroparous (spawning in several batches), asynchronous (having oocytes in all developmental stages), and indeterminate (new oocytes can develop in the female and be released during spawning) (Medina et al 2002, Murua & SaboridoRey 2003. The absolute fecundity is much lower in bullet tuna and albacore than in bluefin tuna due to their smaller size of the former, even though their relative batch fecundity is similar, around 59 eggs g −1 (Medina et al 2007, Saber et al 2011. Because of its large size, bluefin tuna can also store larger amounts of energy for reproduction and tolerate a broader range of environmental scenarios (Sharp 2001, Medina et al 2002 than the other 2 species.…”

“…Bluefin tuna most probably use stored energy to develop the gonads (Medina et al 2002). Their spawning events are sporadic, and the adults stay in the spawning ground no more than 2 wk, leaving soon thereafter (Gunn & Block 2001, Medina et al 2007), …”

Reproductive strategies of the marine fishes from the southwest Atlantic Ocean: an application of histological and image processing techniques

“…So far, the model-based stereology is the most common procedure to estimate the number of oocytes in fish (GANIAS et al, 2014). This includes oocytes in primary growth, vitellogenesis and atresia, as well as post-ovulatory follicles (ISAAC-NAHUM et al, 1988;GREER WALKER et al, 1994;WÖHLER, 1994;BROMLEY et al, 2000;COWARD;BROMAGE, 2002;MEDINA et al, 2002;COOPER et al, 2005;MEDINA et al, 2007;ALONSO-FERNÁNDEZ et al, 2008;SABER et al, 2011;HASLOB et al, 2013;GARCÍA-SEOANE et al, 2014;KNAPP et al, 2014;NEVES et al, 2014;VILLEGAS-RÍOS et al, 2014). Although, it should not be used to count cells of irregular shape and/or highly variable size, e.g.…”

“…Thus, stereological methods have been widely applied in fishery science as an accurate tool for estimating the number of vitellogenic oocytes/ fecundity from ovarian histological preparations (COWARD;BROMAGE, 2002;MEDINA et al, 2007).…”