Natural and fishing mortalities affecting eastern sea garfish, Hyporhamphus australis inferred from age-frequency data using hazard functions
Broadhurst, Matt K. | Kienzle, M. (Marco) | Stewart, John
Estimates of age-specific natural (M) and fishing (F) mortalities among economically important stocks are required in age-structured models 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 (2004–2015) 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 was characterised by: (i) the escape of fish aged 0–1 years increasing from approximately 90 to 97% (assumed to be a result of a mandated increase in stretched mesh opening from 25 to 28mm); (ii) full selectivity among older age groups; (iii) a constant M of 0.52 ± 0.06 year⁻¹; 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.
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