Age and maturity of spiny dogfish (Squalus acanthias) under increased fishing pressure in New Zealand

Globally, shark populations can be susceptible to overexploitation from fishing due to their conservative life histories (e.g., late maturity, slow growth, long gestation periods, and low fecundity) that result in overall low biological productivity. Spiny dogfish (Squalus acanthias) is one of six shark species managed under the New Zealand Quota Management System and it is a primary bycatch species in some of the deep-water fisheries. Despite its prevalence as a bycatch species, most of our biological and ecological knowledge for New Zealand spiny dogfish is dated. The objective of this study was to assess the age, size, and maturity of spiny dogfish in the New Zealand region and assess any changes over time since New Zealand spiny dogfish age and growth was last determined in the 1980s. A total of 119 adult/juvenile spiny dogfish and 15 pups were collected from the Chatham Rise in January 2022 (41 males and 76 females). Spiny dogfish lengths ranged between 54.1–85.8 cm total length (TL), and 52.1–92.8 cm TL for males and females, respectively. Spiny dogfish were aged by counting growth bands on the second dorsal fin spine. Of the 112 spines, 93.3% had readable banding patterns, inter-reader bias in age estimates was detected, but count precision was relatively high between reader age estimates (CV= 10.04). Growth was best described by the von Bertalanffy growth model, and a significant difference in the growth rates for each sex was determined. Von Bertalanffy growth parameters were: Linf= 61.55 cm, K= 0.72 cm, t0 = -0.64 years, and Linf= 76.74 cm, K= 0.14 cm, and t0= -0.64 years for males and females, respectively. Spiny dogfish were sexually dimorphic with females growing to a larger size than males. Females attained 50% maturity at 14 years old and 65.5 cm TL, males reached length at maturity at 51.5 cm TL, but age at maturity was unable to be determined using the same method due to lack of immature males and was instead estimated to be between 5-16 years old. The oldest observed spiny dogfish were estimated at 20 and 31 years for males and females, respectively. Comparison with historical data indicated lower maximum lengths, length at 50% maturity and length-at-age estimates in 2022 compared to 1986, suggesting decreased growth and highlighting the potential impact of commercial fishing on this species, given its low productivity. Despite sampling bias, the results of this study emphasize the potential vulnerability of spiny dogfish populations to overexploitation and the need for further research on the life-history characteristics of the species.