P Burch

Survey estimates of fishable biomass following a mass mortality in an Australian molluscan fishery

Mass mortality events are relatively uncommon in commercially fished populations, but when they occur, they reduce production and degrade ecosystems. Observing and documenting mass mortalities is simpler than quantifying the impact on stocks, monitoring or predicting recovery, and re-establishing commercial fishing. Direct survey measures of abundance, distribution and harvestable biomass provide the most tenable approach to informing decisions about future harvests in cases where stock collapses have occurred because conventional methods have been disrupted and are less applicable. Abalone viral ganglioneuritis (AVG) has resulted in high levels of mortality across all length classes of blacklip abalone, Haliotis rubra Leach, off western Victoria, Australia, since May 2006. Commercial catches in this previously valuable fishery were reduced substantially. This paper describes the integration of research surveys with commercial fishermens knowledge to estimate the biomass of abalone on AVG-impacted reefs. Experienced commercial abalone divers provided credible information on the precise locations of historical fishing grounds within which fishery-independent surveys were undertaken. Abalone density estimates remained low relative to pre-AVG levels, and total biomass estimates were similar to historical annual catch levels, indicating that the abalone populations have yet to adequately recover. Survey biomass estimates were incorporated into harvest decision tables and used with prior accumulated knowledge of the populations to determine a conservative harvest strategy for the fishery.

Estimating pup production in a mammal with an extended and aseasonal breeding season, the Australian sea lion (Neophoca cinerea)

Abstract Context. The Australian sea lion population at Seal Bay Conservation Park, South Australia, was estimated to be declining at a rate of 1.14% per breeding season, on the basis of maximum counts of live pups in each of 13 breeding seasons (Shaughnessy et al. 2006). The reliability of the pup-production estimates used to identify this decline is uncertain. Aims. Our aims were to obtain representative and repeatable estimates of pup production and to assess the current rate of decline. Methods. We compared four estimates of pup abundance over five breeding seasons (2002–03, 2004, 2005–06, 2007, 2008–09), including the count of cumulative new births, the maximum live-pup count, the number of pups given passive integrated transponder (PIT) tags, and mark–recapture methods using the Petersen estimate. Key results. A total of 90% of pup births occurred over a mean of 124 days (s.d. = 14). Final estimates of pup production (from the largest of the four estimation methods used) in the five seasons were 227 (CL 221–239), 288 (CL 273–302), 219 (NA), 260 (CL 254–272) and 268 (CL 268–269). The average estimate of pup mortality was 28.6% (s.d. = 6.3%). The decline in the population at Seal Bay over 17 breeding seasons on the basis of maximum counts of live pups was 0.51% per year or 0.76% per breeding season. However, this trend was not based on best estimates of pup production. On the basis of final estimates for the last five breeding seasons, there is no declining trend. Conclusions. The count of cumulative new births was the most reliable measure of pup production; the Petersen mark–recapture estimate provided a check for accuracy and confidence limits about the estimate. Implications. The actual rate of change and the expected trajectory of the Seal Bay population remain uncertain. Ongoing monitoring is a priority for this site, using the reliable methods of estimating pup production identified in the present study.

Use of the Daily Egg Production Method for Stock Assessment of Sardine, Sardinops sagax; Lessons Learned over a Decade of Application off Southern Australia

Analyses of data collected over a decade off southern Australia confirm that estimates of spawning biomass of Sardinops sagax obtained using the daily egg production method are imprecise and suggest that if inappropriate analytical methods are used, estimates may also be biased. Spawning biomass estimates are most affected by variation in mean daily egg production (P0), spawning area, and spawning fraction. The log-linear mortality model (with one egg added to each day class of eggs at each positive site) should be used to estimate P0 because it fits strongly over-dispersed sardine egg density data better and provides more logically consistent and precautionary estimates of P0 than the exponential mortality model or generalized linear models. Most generalized linear models produced inflated estimates of P0 when egg data were strongly over-dispersed. The area surveyed should be sub-divided into a large number (e.g., 300) of similar sized grids. The Voronoi natural neighbor method should be used to calculate grid size for estimation of the spawning area because it reduces subjectivity in sub-division of the sampling area. Potential biases in spawning fraction can be minimized by calculating this parameter using data from all three stages of post-ovulatory follicles (combined). Priorities for future research are identified.

Combining passive integrated transponder tags with conventional T-bar tags to improve tag reporting rates in a rock lobster trap fishery

Abstract Tag reporting rate is an important parameter required for estimating fishing and natural mortality in fished populations. It is an extremely difficult parameter to estimate and can vary both during the fishing season and between fishing years. Improving tag reporting rates has been identified as a cost‐effective way of improving precision in fishing and natural mortality estimates derived from tagging data. In this study we evaluated the use of passive integrated transponder (PIT) tags to improve tag reporting rates in a lobster (Jasus edwardsii) trap fishery. To minimise the risk of PIT tag ingestion by consumers, we tested insertion of the PIT tag into the base of the antennae, the base of the walking leg, and the base of the telson. The base of the antennae proved to be the best option for lobster health although impacts were observed at all tag insertion sites. We further developed and tested a hybrid tag that combined a conventional external T‐bar tag with a PIT tag embedded within the shaft. PIT tag scanners were positioned in bottlenecks in the capture process so that all lobsters retained on board the vessel were passed through a scanner. Thus, the tag reporting rate was expected to be 100% for the subsample of the fleet fitted with scanners when scanners were operational. Scanner malfunction was a major issue and further engineering solutions are required to ensure that scanners are reliable when working in harsh marine conditions such as on‐board fishing vessels. Advantages of the hybrid tag are: the ability to be detected by both automatic detectors and fishers; known tag‐induced mortality and tag loss rates from previous research; and PIT tagging without risk of human ingestion.

Estimating Species-Specific Catch Rates in a Mixed-Species Dive Fishery

ABSTRACT Catch-per-unit-effort (CPUE) is frequently used as a measure of relative abundance in fisheries stock assessment. Determining reliable estimates of species-specific CPUE is more challenging in multispecies, rather than single-species, fisheries because identification of appropriate effort data for each species is often difficult. Divers in the South Australian abalone fishery can harvest blacklip (Haliotis rubra) and greenlip (Haliotis laevigata) abalone simultaneously, but report only a single value for daily fishing effort. This is problematic because total allowable commercial catches are set for each species following species-specific stock assessments in which CPUE is a key index of relative abalone abundance. To provide an evidence-based approach to the identification of the most appropriate CPUE estimation method for ongoing assessment of the fishery, we assessed six diverse CPUE estimation methods for estimating annual, species-specific CPUEs using 30 y of data. The candidate CPUE estimation methods yielded relative CPUE time series with similar temporal trends throughout the 30-y period. These relative CPUE estimates each had low coefficients of variation and were highly correlated with one another, requiring consideration of other factors to determine a preferred method. Using a catch-weighted estimate of CPUE (CPUEwt) overcomes many of the problems associated with using the other five methods tested. Specifically, CPUEwt (1) weights each daily catch and effort objectively; (2) removes the need to “subset” the data subjectively, which ensures that data availability and representation are not reduced by arbitrary rules; and (3) is relatively simple to explain to stakeholders and can be applied consistently to greenlip and blacklip abalone at multiple spatial scales across the fishery. Although the requirement to estimate species-specific catch rates in mixed-species dive fisheries is rare, our analyses demonstrate that CPUEwt could provide a robust measure of species-specific CPUEs across other diverse multispecies fisheries.

Pup numbers of the Australian sea lion (Neophoca cinerea) at The Pages Islands, South Australia, over two decades

Abstract. The Australian sea lion is an Australian endemic, restricted to South Australia and Western Australia, with 86% of the population in South Australia. It was listed under the Commonwealth Environment Protection and Biodiversity Conservation Act as Vulnerable in February 2005, and the International Union for the Conservation of Nature has listed it as Endangered. Sea lions are taken as bycatch in the gill-net fishery for school shark and gummy shark, and the risk of extinction of breeding colonies is high even from low levels of bycatch. We assessed trends in pup population size at The Pages Islands, a large breeding colony in South Australia. Pup abundance was estimated by direct counting of live and dead pups; the maximum count in each breeding season was used for trend analysis. The average of direct counts of pups in 14 breeding seasons between 1989–90 and 2009–10 was 473 (s.d. = 58.4). There was no trend in pup numbers, contrasting with two other large colonies: Seal Bay, Kangaroo Island (decreasing), and Dangerous Reef (increasing since 2000). The Australian Sea Lion Management Strategy of the Australian Fisheries Management Authority aims to reduce sea lion bycatch in the shark fishery; a key item is a fishery closure around each breeding colony in South Australia. Implementation of the closure around The Pages should lower the risk of bycatch of its sea lions with foraging areas that previously overlapped with the fishery and should allow the colony’s population size to increase.

A modelled cost-benefit analysis of hybrid PIT and conventional tagging scenarios

Abstract Tag recovery rate is an important parameter for estimating exploitation and natural mortality in fished populations. Passive integrated transponder (PIT) technology can record 100% of PIT tagged animals passing within the detection limit of a PIT tag scanner. However PIT tags and PIT tag scanners are expensive compared with conventional visual tags and PIT tags are not detectable without scanners. We used simulation to evaluate the hybrid PIT tag which has the PIT tag incorporated into a conventional tag allowing both electronic detection capability by a scanner and visual detection by fishers. Simulated estimates of the precision and accuracy of exploitation and natural death rates for two lobster fishery management regimes: a 7‐month season for both sexes and a fishery with a 10‐ and 6‐month season for males and females, respectively, were used to determine the benefit of PIT tags. For a project budget of AU#DL200,000, hybrid PIT tags and 10% of the fleet being equipped with scanners produced more precise and accurate estimates of exploitation rate and natural death rate until tag reporting rate by fishers exceeds 40% for the two‐gender management regime and 90% for the 7‐month combined sex regime. Increasing the number of scanners to 20% of the fleet resulted in fewer hybrid PIT tags being inserted for the same cost and did not improve the precision or accuracy of estimates. Increasing the number of lobsters tagged by tagging during a higher catch rate period improved the precision of exploitation and natural death rate estimates at lower tag reporting rates for the conventional tag scenario, but did not alter the tag reporting rate required to make conventional tagging more beneficial than hybrid PIT tagging. Increased tag reporting rate by the 90% of the fleet that were not equipped with scanners had no significant impact on the precision or accuracy of estimates for either management regime.