Why do fish reach first maturity when they do?

Global patterns and drivers of fish reproductive potential on coral reefs

Fish fecundity scales hyperallometrically with body mass, meaning larger females produce disproportionately more eggs than smaller ones. We explore this relationship beyond the species-level to estimate the "reproductive potential" of 1633 coral reef sites distributed globally. We find that, at the site-level, reproductive potential scales hyperallometrically with assemblage biomass, but with a smaller median exponent than at the species-level. Across all families, modelled reproductive potential is greater in fully protected sites versus fished sites. This difference is most pronounced for the important fisheries family, Serranidae. When comparing a scenario where 30% of sites are randomly fully protected to a current protection scenario, we estimate an increase in the reproductive potential of all families, and particularly for Serranidae. Such results point to the possible ecological benefits of the 30 × 30 global conservation target and showcase management options to promote the sustainability of population replenishment.

Juvenile-to-adult transition invariances in fishes: Perspectives on proximate and ultimate causation

To bridge physiological and evolutionary perspectives on size at maturity in fishes, the authors focus on the approximately invariant ratio between the estimated oxygen supply at size at maturity (Q_m ) relative to that at asymptotic size (Q_∞ ) among species within a taxonomic group, and show how two important theories related to this phenomenon complement each other. Gill-oxygen limitation theory proposes a mechanistic basis for a universal oxygen supply-based threshold for maturation, which applies among and within species. On the contrary, the authors show that a generalisation of life-history theory for the invariance of size at maturity (L_m ) relative to asymptotic size (L_∞ ) can provide an evolutionary rationale for an oxygen-limited maturation threshold (Q_m /Q_∞ ). Extending previous inter- and intraspecific analyses, the authors show that maturation invariances also occur in lake whitefish Coregonus clupeaformis (Mitchill 1818), but at both scales, theory seems to underestimate the value of the maturation threshold. They highlight some key uncertainties in the model that should be addressed to help resolve the mismatch.

The ratio of length at first maturity to maximum length across marine and freshwater fishes

The prevailing determinant of maturation in fishes is thought to be a redirection of energy from growth to reproduction. Instead, the Gill Oxygen Limitation Theory predicts that maturation, and thus reproduction, is induced when a fish reaches a critical ratio of oxygen supply to demand (Q_m /Q_maint ). The consistency of this critical ratio has been previously documented in many fishes, but a broader test was lacking. In this study, the authors assess if this critical ratio is consistent across 132 unique fish species, as measured by the slope of the relationship between L_max ^D and L_m ^D , where L_max is the maximum length reached in a given population, L_m is the mean size at first maturity in that population and D is a gill-related exponent which renders the L_max ^D /L_m ^D ratio equivalent to the Q_m /Q_maint ratio. The authors found that across all species, the L_max ^D /L_m ^D ratio was 1.40 (95% c.i. 1.38-1.42), which was not significantly different from that previously estimated across other species groups (1.35, 95% c.i. 1.22-1.53), especially when phylogenetic relationships were considered (1.25, 95% Bayesian credible interval 1.09-1.40). The consistency of the L_max ^D /L_m ^D ratio across taxa, which expresses the difference in metabolic rate at maturity and maximum size, suggests that the scaling of gill surface area is the factor that underlies this ratio, and which triggers the maturation in fishes.

Length–Weight Relationships and Growth Parameters of Common and Leafy Seadragons (Syngnathidae) from a Public Aquarium

The length–weight relationships (LWR) of the common seadragon Phyllopteryx taeniolatus (Lacepède, 1804) and the leafy seadragon Phycodurus eques (Günther, 1865), Syngnathidae, are presented in this paper, based on specimens raised in the Birch Aquarium at Scripps, La Jolla, California. Furthermore, we used the length at known age of 40 specimens of common and 15 leafy seadragons to estimate the parameter of the von Bertalanffy growth function (VBGF) for these species. Some of the pros and cons of the newly proposed length type used, ‘scalene length’ are presented. The parameters of the LWR and the VBGF parameters are discussed, with an emphasis on the rearing conditions, the peculiar anatomy of seadragons, especially of P. eques, and on the Gill-Oxygen Limitation Theory (GOLT).