Blue Cod, Parapercis Colias, Maturity, Fecundity, Sex Change, and Potential Drivers of Sex Ratio in The Marlborough Sounds
Blue cod, Parapercis colias (Pinguipedidae), are widely considered to be the most important recreational finfish species in the South Island. However, blue cod have been declining in abundance in the Marlborough Sounds for many years, and currently, sex ratios are highly male- biased. Blue cod are believed to be a socially structured hermaphroditic species whose populations are prone to local depletion. A potting survey in the Marlborough Sounds was carried out by NIWA in 2013, and along with environmental measurements, 3247 blue cod were measured, weighed, sexed, and stage of sexual maturity was classified macroscopically at sea. Gonads from a subsample of these fish were removed and preserved. In Chapter Two, the preserved gonads were processed histologically and a species-specific histological maturity key was developed. Histological and macroscopic maturity classifications and length-at-maturity estimates were compared. Additionally, estimates of spawning frequency and batch fecundity were made using histological and gravimetric methods. In Chapter Three, possible drivers of sex ratio were investigated using the survey data. Density, large male influence, and environmental factors were considered. In Chapter Four, the feasibility of using digital imaging software to age blue cod otoliths was investigated using the OtolithM application in ImagePro Premier. There was poor agreement between macroscopic and histological maturity classifications (20%, overall). Macroscopic methods overestimated the proportion of mature fish at length in the larger sample, which led to biased length-at-50% maturity (L₅₀) estimates. Macroscopic L₅₀ estimates differed markedly from histology estimates. Using histological data, male L₅₀ was 26 cm TL. Histology indicated that there was no length at which 100% of females were mature. Therefore, a three-parameter capped logistic model was used. Histologically, female L₅₀ was 23.6 cm TL, and the Cap was 0.78, indicating the proportion of mature females reached an asymptote at 0.78. Spawning frequency was 4.6 days, and mean relative batch fecundity was 6.5 hydrated oocytes per gram body weight (SD = 3.3). Hermaphroditism was confirmed for blue cod and was macroscopically identifiable. The analyses in Chapter Three indicated that density had some effect on sex ratio, and large males influenced local sex ratios. Finally, the imaging software could not accurately estimate age compared to an expert reader, and it produced highly variable age estimates. This research found that the macroscopic maturity classifications for blue cod were inaccurate, and revision of the macroscopic key is suggested. The biased estimates of L₅₀ from macroscopic data could lead to biased estimates of spawning stock biomass (SSB). Batch fecundity was markedly lower than the previously reported estimates. The finding of a macroscopically identifiable hermaphroditic stage suggested that ‘hermaphrodite’ should be added as a sex class in the macroscopic key. From the GLM in Chapter Three, in more dense populations, the proportion of males increased. This may have been from changes to male mating strategies, or density may influence the occurrence of primary and secondary males. Finally, areas with males > 45 cm TL had a higher proportion of females, suggesting that large males should be protected in order to help balance sex ratios.