Natural and fishing mortalities affecting eastern sea garfish, Hyporhamphus australis, inferred from age-frequency data using hazard functions

TL;DR

This study used hazard functions to estimate natural and fishing mortalities of eastern sea garfish, revealing increased escape rates for young fish, constant natural mortality, and declining fishing mortality over time, aiding sustainable fishery management.

Contribution

It applies hazard functions to age-frequency data to accurately estimate mortality rates in eastern sea garfish, demonstrating their utility for resource management.

Findings

Escape of young fish increased with mesh size regulation

Natural mortality rate estimated at 0.52 per year

Fishing mortality declined from 2004 to 2015

Abstract

Estimates of age-specific natural (M) and fishing (F) mortalities among economically important stocks are required to determine sustainable yields and, ultimately, facilitate effective resource management. Here we used hazard functions to estimate mortality rates for eastern sea garfish, Hyporhamphus australis, a pelagic species that forms the basis of an Australian commercial lampara-net fishery. Data describing annual (2004 to 2015) age frequencies (0-1 to 5-6 years), yield, effort (boat-days), and average weights at age were used to fit various stochastic models to estimate mortality rates by maximum likelihood. The model best supported by the data implied: (i) the escape of fish aged 0-1 years increased from approximately 90 to 97% as a result of a mandated increase in stretched mesh opening from 25 to 28 mm; (ii) full selectivity among older age groups; (iii) a constant M of 0.52 +- 0.06 per year; and (iv) a decline in F between 2004 and 2015. Recruitment and biomass were estimated to vary, but increased during the sampled period. The results reiterate the utility of hazard functions to estimate and partition mortality rates, and support traditional input controls designed to reduce both accounted and unaccounted F.