Improving age, growth, and maturity estimates for aseasonally reproducing chondrichthyans

Abstract

Many species of chondrichthyans (sharks, rays and chimaeras) reproduce throughout the year, lacking a distinct seasonality in the timing of their reproduction with young born throughout the year. Since age determination in fish usually relies on counting regularly deposited growth increments on hard body parts (e.g. vertebrae, spines, otoliths) the age of an aseasonally reproducing animal after the first growth increment is formed is unknown. We explored the implications of this for the estimation of growth and maturity parameters using the milk shark, Rhizoprionodon acutus, a small, tropical, shark that reproduces aseasonally off northern Australia. Since R. acutus grows rapidly after birth, not accounting for an aseasonal reproductive cycle led to unrealistic projections of growth for this species. We describe a simple method for adjusting individual ages that improves projections of growth and reduces the level of error around growth parameter estimates. This is compared with growth estimates when ages are left unadjusted or adjusted to a mean value to account for aseasonal reproduction. Using back calculated length-at-age data, model-averaged growth across five models gave an asymptotic size (L∞) of 859 mm for females and 821 mm for males. Standard von Bertalanffy growth parameters were L∞ = 861 mm, K = 0.63, L0 = 423 mm for females and L∞ = 821 mm, K = 0.94, L0 = 424 mm for males. The oldest female and male were 8.1 and 4.5 years old, and the largest female and male measured were 940 and 931 mm. The size at which 50% of females and males were mature was 780 and 742 mm. The age at which 50% of females and males were mature was 1.8 and 1.1 years of age. Incorporating the effects of an aseasonal reproductive cycle into growth analysis is an important step for maximising the accuracy of age and growth results and is especially important for small, tropical species such as R. acutus that complete much of their growth in the first year of life.