Estimates of biomass

One of the highest research priorities for Snapper is the development of reliable fishery-independent methods to monitor trends in population size and biomass (research theme 2). This need is driven by:

• the influence of management changes on fishery statistics
• hyperstability which occurs when relationships between catch rates and abundance diverge
• absence of reliable fishery-dependent data – for example, due to fishery closures.

Traditionally, catches and catch rates (catch per unit effort – CPUE) have been used as a relative index of abundance for many fishery species. This is based on the assumption that an increasing catch rate reflects a higher abundance of fish, and a decreasing catch rate reflects a lower abundance of fish. However, this relationship can be influenced by numerous factors, including changes to catch limits from fishery management, such as daily trip limits and spatial closures.

Hyperstability is another factor which occurs when the relationship between catch rates and abundance diverge, such that high catches and catch rates can persist although the population is declining – the hyperstability 'masks' the decline of the population.

When this occurs, catches and catch rates may only decline once the population has become depleted. A variety of factors contribute to hyperstability, such as higher fishing efficiency through improved technology (for example: sonar, GPS, radar, communications) and aggregating behaviour linked to spawning and migration that concentrates fish.

Estimates of biomass involves developing and refining 4 different methods to estimate population size:

• daily egg production method (DEPM)
• hydroacoustic surveys
• close-kin mark-recapture (CKMR)
• fishery modelling.

Refinement of daily egg production method (DEPM) methodology

Project commencing late 2023

Over the last 30 years, the daily egg production method (DEPM) has been used to estimate the spawning biomass of Snapper in South Australia, Western Australia and New Zealand. The underlying principle of the DEPM is that spawning biomass can be estimated from the relative abundance of eggs in the water column and the number of eggs produced by spawning fish.

This project will review the DEPM methodology used to estimate spawning biomass for Snapper and explore methodological and statistical approaches to enhance confidence in biomass estimates, including:

• evaluating various methods to sample Snapper eggs from the water column
• assessing different approaches to determine total daily egg production
• comparing estimates of biomass with a concurrent hydroacoustic survey.

Development and application of hydroacoustic surveys

Project commences late 2023

Hydroacoustic techniques use underwater sound signals to identify fish and are commonly used to survey pelagic fish species, for example Sardines and Anchovies. Acoustic surveys have the capacity to cover large areas relatively quickly and gather information over a wide range. More recently, hydroacoustic techniques have been developed to survey for demersal fish species – species that generally occupy the bottom third of the water column – although this is associated with several challenges.

This study will explore the feasibility of hydroacoustic surveys to estimate the biomass of Snapper aggregations in South Australia's gulfs using the active acoustic methodology developed in Western Australia – see Estimating abundance of fish associated with structured habitats by combining acoustics and optics.

The study, led by the CSIRO, will investigate the current limitations of the approach and refine the methodology to assess if estimates of biomass from this technique are suitable for formal stock assessment.

Evaluation of close-kin mark-recapture (CKMR) for Snapper

Project commences July 2024

Close-kin mark-recapture (CKMR) involves estimating the size of a spawning population based on the number of closely related individuals in a sample, such as parent-offspring pairs or half-sibling pairs. For example, given a random sample of fish from a population, more closely related pairs would be expected in a smaller population, while less related pairs are expected in a larger population.

CKMR can be used to estimate abundance, natural mortality, and fecundity with high precision and is independent of fishery statistics. It is a reasonably new fishery assessment tool which is increasingly popular and has been successfully applied to several commercial species including southern bluefin tuna and school shark.

This project will undertake a scoping study, led by the CSIRO – world leaders in the field – to evaluate the logistical and economic feasibility of CKMR as a method to estimate the biomass of Snapper.

Enhance the Snapper stock assessment model

Project in progress

The Snapper stock assessment model (SnapEst) uses numerous biological and fishery datasets to provide:

• annual estimates of biomass
• exploitation rate
• recruitment for each stock of Snapper in South Australia.

The model was initially developed in the early 2000s and has evolved to incorporate new datasets through time such as spawning biomass estimates from the DEPM.

The proposed project to enhance the stock assessment model involves 3 components:

• an external review of the existing model by an independent expert
• investigating the possibility of integrating new datasets developed through the Snapper Science Program
• developing a projection model to predict potential future trends in the population.

An external review is currently being finalised and the recommendations will guide the structure of the project. The projection model will use the estimates of recruitment from juvenile Snapper surveys to forecast trends in recruitment and adult biomass.