Improved estimates of age, growth and reproduction for the regionally endemic Galapagos sailfin grouperMycteroperca olfax(Jenyns, 1840)

Abstract

The Galapagos Sailfin grouper, Mycteroperca olfax, locally known as bacalao and listed as vulnerable by the IUCN, is culturally, economically, and ecologically important to the Galapagos archipelago and its people. It is regionally endemic to the Eastern Tropical Pacific, and, while an important fishery resource that has shown substantial declines in recent years, to date no effective management regulations are in place to ensure the sustainability of the Galapagos fishery for this species. Previous estimates of longevity and size at maturity for bacalao are inconsistent with estimates for congeners, which brings into question the accuracy of prior estimates. We set out to assess the age, growth, and reproductive biology of bacalao in order to provide more accurate life history information to inform more effective fisheries management for this species. The oldest fish in our sample was 21 years old, which is 2–3 times greater than previously reported estimates of longevity. Parameter estimates for the von Bertalanffy growth function (k = 0.11, L∞ = 110 cm TL, and to = − 1.7 years) show bacalao to grow much slower and attain substantially larger asymptotic maximum length than previous studies. Mean size at maturity (as female) was estimated at 65.3 cm TL, corresponding to a mean age of 6.5 years. We found that sex ratios were extremely female biased (0.009 M:1F), with a large majority of the individuals in our experimental catch being immature (79%). Our results show that bacalao grow slower, live longer, and mature at a much larger size and greater age than previously thought, with very few mature males in the population. These findings have important implications for the fishery of this valuable species and provide the impetus for a long-overdue species management plan to ensure its long-term sustainability.

So Long and Thanks for All the Fish: Overexploitation of the Regionally Endemic Galapagos Grouper Mycteroperca olfax (Jenyns, 1840)

“…Detailed descriptions of the sampling methodology can be found in [16,31]. However, due to logistics constraints, data for the January-February 2013 period covered only the island of Santa Cruz.…”

“…The indicators were calculated as follows: 1) L m : Size at first maturity for bacalao was recently estimated at 65.3 cm TL [16]. 2) L opt : optimum length at catch was estimated using the empirical relationships between optimum length and ultimate maximum length (L ∞ ) derived from the equation (L opt = 10 1.0421*log 10 (L ∞ )-0.2742 [36], which was then used to estimate the optimum length interval as ± 10% of L opt [35].…”

“…While an estimate for k exists for bacalao [16], no such estimate exists for an unfished population of bacalao. In order to obtain an estimate for the M/ k parameter which is needed as an input to the model, we used an estimate of m from a congener ( Mycterperca venenosa ) m = 0.29 [42].…”

“…We estimated the average time required for a new generation to replace the last generation by calculating the mean generational turnover rate () following the methods proposed by [47] and applied to fishes by Depczynski and Bellwood (2006) [48], as given by the equation: where AM is age at maturity in females, and T max is maximum age, both of these estimates were obtained from [16]. …”

“…Early stock assessments determined bacalao harvest to be at sustainable levels [6,15], yet, Usseglio et al (2015)[16] showed that bacalao grow slower, live longer, and mature at a greater age than previously assumed, which makes this species much more susceptible to overfishing than formerly thought. While bacalao comprised nearly all of the finfish landings in 1940s, it made up < 17% in recent years [6,17].…”