Why do fish reach first maturity when they do?

Abstract

Why do fish reach first maturity when they dothe question posed by the title of this contributionis usually not asked by fish biologists. This could be because there seem to be obvious answers, i.e., they reach first maturity when they become adults or they have evolved that way. Nonetheless, a moment of reflection suffices to realize that such "explanations" are either circular (because reaching first maturity means that a fish becomes an adult) or non-explanatory (because everything that fish can do results from their evolution).The explanation provided here relies on the concepts and parameters in Table 1.Textbooks usually attempt to answer the question in the title question by describing a process supposedly triggered by environmental "stimuli" or "input" experienced at the onset of the spawning season, passed on to the hypothalamus, and thence to a hormonal cascade (Figure 1). Thus, Bhattacharya (1992) wrote the following as caption to what is now Figure 1a: "Hormonal control of reproduction in fishes: Environmental stimuli like photoperiod, temperature, etc., are received by exteroreceptors [sic] and reach the brain (hypothalamus). Environmental cues result in secretion of GnRH from the brain which in turn causes the release of pituitary GtH. GtH acts on the testis or ovary and stimulates the production of steroid hormones. An exceptional steroid hormone, 17a, 20β-diOHprog, released from the testis and ovary induces the final maturation of germ cells leading to spermiation and ovulation (spawning)," and Pankhurst (2016) provided

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

“…Distinct fish families usually have a constant ratio of L m /L ∞ (Beverton & Holt, 1959), which has misled some authors (e.g., Charnov, 1993) into perceiving this ratio as "invariant," but which can actually be seen as an approximate expression of a more basic Q m / Q maint ratio (Pauly, 2021a(Pauly, , 2021b. When the relative oxygen supply (mass-specific metabolic rate or mass-specific gill surface area) in growing juvenile declines to 1.3-1.4 times their maintenance metabolism (Q maint ), i.e., when Q m is reached, the stress that this induces may make them start to register environmental stimuli, triggering the hormonal cascade leading to maturation and spawning (Pauly, 2021b).…”

“…The conclusion seems unavoidable that late-stage juveniles must reach an individual internal state that makes them take notice of environmental stimuli for maturation (Pauly, 1984(Pauly, , 2021b. Pauly (1984) proposed that this internal state of readiness is generated by the fact that as fishes grow in weight (i.e., in three dimensions), their gill respiratory area, as a surface, grows in a lower dimension, as summarized by the equation:…”

“…In any case, for fish to be able to devote part of the energy acquired through metabolism to generating their first gonadal products, the internal state triggering maturation must occur before their relative oxygen supply equals their relative oxygen demand (Pauly, 2021b). The hypothesis here is that this occurs when their metabolic rate (Q m ) relative to their maintenance metabolism (Q maint )…”

“…drops to a critical level (Q m /Q maint ), inducing a stress that causes these fish to respond to environmental stimuli, which also explains why long-lived fishes can spend decades as pre-adults, i.e., without spawning, although they seasonally exposed to the same stimuli as the (spawning) adults (Pauly, 2021b).…”

“…This notion, however, does not account for the fact that the juveniles of slow‐growing, longer‐lived fishes, e.g ., sturgeons (Dadswell, 1979), pass through dozens of potential spawning seasons without stimuli‐induced maturation and spawning. The conclusion seems unavoidable that late‐stage juveniles must reach an individual internal state that makes them take notice of environmental stimuli for maturation (Pauly, 1984, 2021b).…”