Factors affecting larval and juvenile snapper growth and condition in Parengarenga Harbour

Abstract

The purpose of this study was to determine factors affecting the larval and juvenile growth rates and condition of snapper (Pagrus auratus) in Parengarenga Harbour, a known nursery ground for P. auratus. P. auratus were only observed over seagrass habitats, so the study sites were all sites of varying density of seagrass. Parengarenga Harbour is made up of four arms surrounded by varied land use, including pasture and pine plantation logging. This divided up the study site into blocks of varied anthropogenic influence. In order to determine growth rates, otoliths were extracted from a total of 114 juvenile P. auratus. Otolith increments were counted and measured, and the Modified Fry Model was used to back-calculate growth through the larval and juvenile phases (analysed separately). Condition of P.auratus were analysed using the Fulton Condition Index to determine the differences in condition of different sized fish. The results of this study suggest that the time of spawning is an important factor in driving juvenile growth rate, where those born earlier in the season have faster growth rates. The highest densities of P. auratus were collected over sites of medium to high density seagrass. Higher growth rates of juvenile P. auratus were observed in the arm considered to be the most pristine, while lower growth rates were associated with pasture and pine plantation logging sites. Condition was variable between the blocks, and no significant differences were observed. Larval size was driven largely by its age, whereby as age increased, so did larval size. Juvenile size was driven by size of the fish at settlement, rate of larval growth and condition. Understanding the early life history characteristics is important in being able to predict, and better understand recruitment. In order to determine what drives high and low recruitment to the fishable population, it is important to understand what drives the success of those who grow to survive until recruitment, as mortality in the early stages of life is high. As well as understanding recruitment, understanding the interactions between juvenile P. auratus and its habitats supports the idea of ecosystem based management. Seagrass habitats are declining at a global scale, so it is important to emphasise their importance as nursery habitats for juvenile fish.