Probabilistic reaction norm reveals family-related variation in the association between size, condition and sexual maturation onset in Atlantic salmon (Salmo salar)

This study investigated the relationship between the size, condition, year class, family, and sexual maturity of Atlantic salmon (Salmo salar) using data collected in an aquaculture selective breeding programme. Males that were sexually mature at 2 years of age (maiden spawn) have, on average, greater fork length and condition factor (K) at 1 year of age than their immature counterparts. For every 10-mm increase in fork length or 0.1 increase in K at 1 year of age, the odds of sexual maturity at 2 years of age increased by 1.48 or 1.22 times, respectively. Females that were sexually mature at 3 years of age (maiden spawn) have, on average, greater fork length and K at 2 years of age than their immature counterparts. For every 10-mm increase in fork length or 0.1 increase in K at 2 years of age, the odds of sexual maturity at 3 years of age increased by 1.06 or 1.44 times, respectively. The family explained 34.93% of the variation in sexual maturity among 2-year-old males that was not attributable to the average effects of fork length and K at 1 year of age and year class. The proportion of variation in sexual maturity among 3-year-old females explained by the family could not be investigated. These findings suggest that the onset of sexual maturation in Atlantic salmon is conditional on performance (with respect to energy availability) surpassing a threshold, the magnitude of which can vary between families and is determined by a genetic component. This could support the application of genetic selection to promote or inhibit the onset of sexual maturation in farmed stocks.</p

Retinal adaptations of southern bluefin tuna larvae: implications for culture

We examined Southern bluefin tuna, Thunnus maccoyii, larvae to identify specific retinal adaptations that would indicate both important parameters for culture and larval ecology in the wild. Plastic resin histology, microspectrophotometry and behavioural feeding responses were used to describe visual development. Thunnus maccoyii larvae reflected the visual morphogenesis template commonly observed in many other marine fish species exhibiting indirect development. First-feeding (3 days post-hatching, [dph], 3.4mm standard length [SL]) larvae possessed tightly packed single cone photoreceptors. Rods and twin cones were present in the retina in post-flexion larvae (21 dph, 8.39mm fork length [FL]) with cone mosaic patterns observed in juveniles (30 dph, 21mm FL). Based on the spacing of adjacent photoreceptors and focal length, first feeding larvae had a maximum theoretical visual acuity of 1.23 ± 0.11° that decreased to 0.14 ± 0.02° at 30 dph. Thunnus maccoyii displayed high cell density in the ventral retinal region (cones, bipolar and horizontal cells), a low convergence of cone cells to ganglion cells throughout the retina during larval development (1.1 ± 0.2 to 1.4 ± 0.3 at 3 dph and 30 dph, respectively), and early development of retinal pigment epithelium (RPE) migration. Microspectrophotometry showed twin cone visual pigments maximally sensitive to light in the blue-green part of the visual spectrum (wavelength of maximum absorption [λmax] of 494 nm, 507 nm and 524 nm), and behavioural experiments showed they fed preferentially at these wavelengths. Increased retinal cone densities in the ventral region indicated a localized region specialized for acute vision for prey and predator detection in the upward direction (dorsal plane) at an early age, representing a possible adaptation to life in deeper oceanic waters. The apparent high acuity and photopic sensitivity observed in T. maccoyii is hypothesised to be associated with the ability to feed in low light conditions. This has important practical considerations in determining lighting regimes for culture of T. maccoyii and possibly for other tuna species