Richard McGarvey

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.

An analysis of the sensitivity of stock biomass estimates derived from the daily egg production method (DEPM) to uncertainty in egg mortality rates

In daily egg production methods (DEPMs) of estimating pelagic fish stock biomass, high variance in measured egg mortality rate is the principal source of error. A simple analytic equation was used to test the sensitivity of biomass estimates to daily egg mortality. Biomass estimates were found to be robust with regard to variation in mortality. Other sources of uncertainty can at times be more significant. High sample variance and low sensitivity suggest more optimal survey designs can be achieved by devoting fewer resources to measuring egg mortality rates. A Bayesian approach is suggested.

Evaluating the use of the Daily Egg Production Method for stock assessment of blue mackerel, Scomber australasicus

The present study evaluates the suitability of the Daily Egg Production Method (DEPM) for stock assessment of blue mackerel, Scomber australasicus and assesses methodological options for future applications. In southern Australia, estimates of mean daily egg production were higher for Californian Vertical Egg Tow (CalVET) nets than bongo nets, and in eastern Australia, were higher in October 2003 than July 2004. Estimates of spawning area for southern Australia were three times higher for bongo nets than CalVET nets. Similar estimates of spawning area were obtained using standard (manual) gridding and natural neighbour methods. Large samples and reliable estimates of all adult parameters were obtained for southern Australia. Relatively few spawning adults were collected off eastern Australia. Preliminary best estimates of spawning biomass for southern and eastern Australia were 56 228 t and 29 578 t, respectively, with most estimates within the ranges of 45 000-68 000 t and 20 000-40 000 t respectively. The DEPM is suitable for stock assessment of S. australasicus. Several technical refinements are required to enhance future applications, including: genetic techniques for identifying early stage eggs; a temperature-egg development key; improved methods for sampling adults off eastern Australia; and measurements of the degeneration rates of post-ovulatory follicles at several temperatures.

Methods of Estimating Mortality and Movement Rates from Single-Tag Recovery Data that are Unbiased by Tag Non-Reporting

Single-tag recovery data are widely available for stocks of exploited fish and invertebrates. However, tag non-reporting, short-term tag shedding, and short-term tag-induced mortality cause potentially large bias in population parameter estimates using standard models of exploitation rate (Petersen) and movement (Hilborn). Here, I review estimators of mortality and movement rates that are not biased by these tag loss effects. Mortality can be estimated using the times-at-large of recaptures. The reciprocal of the mean time-at-large of recaptured fish is the maximum likelihood estimate of instantaneous total mortality rate. Chapman refined this to derive a time-at-large mortality rate estimate that is both finite-sample unbiased and minimum variance, thus statistically optimal. With movement estimation, a constant non-reporting rate, by appearing top and bottom in a “recapture-conditioned” model, cancels from the likelihood. Non-reporting rate thereby disappears from the estimator and so is not required to estimate fish movement rates. Tag shedding and tag-induced mortality rates also cancel. These mortality and movement estimators do not use the number of animals tagged and released; using only information from recaptured animals, they are denoted “recapture-conditioned.” Recapture-conditioned estimators offer relatively unbiased single-tag recovery data analysis tools that, to date, have gone largely unnoticed.

Demand-side fishery management: integrating two forms of input control

A fishery management approach is presented which combines (1) a buyback of fishing vessels, and (2) a management tax or fee on seafood going to market. Tax rate by species is set proportional to the extent of overexploitation. Tax revenue is used for several purposes, including a buyback of licenses at free-market price. Advantages and disadvantages of this policy are discussed, with specific comparison to individual transferable quotas (ITQs). This regulatory policy offers advantages (1) for multispecies fisheries, (2) with ecosystem fishery management, (3) where self-funded financing for license buyback is needed, and in place of or together with ITQs (4) where allocation, discarding and highgrading, quota setting, or enforcement of ITQs is problematic.

The importance of conserving recruitment pulses in rock lobster (Jasus edwardsii) fisheries where puerulus settlement is low or highly sporadic

Abstract The commercial fishery for southern rock lobster (Jasus edwardsii) is South Australias most valuable fishery resource worth in excess of AUS

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

100 million annually. Over the past decade, recruitment to the fishable biomass has decreased, which has translated to declines in commercial catch rates. In the Northern Zone fishery, catch per unit effort (CPUE) declined from 1.42 kg/potlift in 1999 to 0.67 kg/potlift in 2008, a decrease of 52%. Puerulus monitoring has been undertaken since 1996, with the period from settlement to recruitment estimated at ~4 years. In 2002, 2005 and 2006, spikes in settlement were recorded, which were predicted to enter the fishery as recruitment pulses in 2006, 2009 and 2010, respectively. Management decisions failed to conserve the 2006 recruitment pulse by setting total allowable commercial catches (TACCs) above actual catch estimates. However, in 2008, a conservative management approach was adopted with the TACC reduced from 470 to 310 t despite 403 t being landed in the 2008 season. In 2009, CPUE increased by 31% to 0.88 kg/potlift while fishing effort decreased by 42%. In 2010, CPUE increased further to 1.07 kg/potlift, the highest since 2000. The study highlights the importance of management decisions that conserve recruitment pulses in order to sustain lobster resources on which fisheries depend.

FIELD TRIALS AND SIMULATIONS OF POINT-NEAREST-NEIGHBOR DISTANCE METHODS FOR ESTIMATING ABALONE DENSITY

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.

Moulting growth of the Australian giant crab, Pseudocarcinus gigas

Abstract We investigated evidence for bias in estimates of abalone density from the point-nearest-neighbor (PNN) diver survey method wherein divers measure distances between abalone and from random points to nearest abalone. Field and simulation tests of the PNN survey method were undertaken. In two plots of a lightly exploited abalone population in South Australia, all the greenlip abalone (Haliotis laevigata Donovan) were enumerated by divers, providing the true density in both study regions. Clustering of abalone was visually evident and quantified by a Hopkins test. The study areas were gridded into 1-m2 quadrats. Divers measured distances from randomly selected grid points to the nearest abalone, and from that nearest abalone to its nearest neighbor. A second set of inter-abalone distances from every fifth tagged abalone were also measured. Two PNN estimator formulas, of Byth (1982) and Diggle (1975), were used to estimate abalone density. The resulting estimates from both PNN estimators were biased, underestimating true (enumerated) density by 18% to 29% and 18% to 55% in the two sites respectively. The Byth estimator showed less underestimation. Clustering of abalone is a likely cause of density underestimation in the two study areas. Simulated PNN surveys in simulated clustered populations quantified both overestimation and underestimation bias. Randomly interspersed individuals (“loners”) reduced density underestimation, and centrally (rather than uniformly) distributed clusters worsened it. Because the spatial distributions of abalone and other invertebrates are often clustered, this strong bias is problematic for the use of PNN as a survey method for estimating density in these populations.

Decision rules for quota setting to support spatial management in a lobster (Jasus edwardsii) fishery

The growth of Australian giant crabs, Pseudocarcinus gigas, has not been previously studied. A tagging program was undertaken in four Australian states where the species is subject to commercial exploitation. Fishers reported a recapture sample of 1372 females and 383 males from commercial harvest, of which 190 females and 160 males had moulted at least once. Broad-scale modes of growth increment were readily identified and interpreted as 0 , 1 and 2 moults during time at large. Single-moult increments were normally distributed for six of seven data sets. Moult increments were constant with length for males and declined slowly for three of four female data sets. Seasonality of moulting in South Australia was inferred from monthly proportions captured with newly moulted shells. Female moulting peaked strongly in winter (June and July). Males moult in summer (November and December). Intermoult period estimates for P. gigas varied from 3 to 4 years for juvenile males and females (80–120 mm carapace length, CL), with rapid lengthening in time between moulting events to approximately seven years for females and four and a half years for males at legal minimum length of 150 mm CL. New moulting growth estimation methods include a generalization of the anniversary method for estimating intermoult period that uses (rather than rejects) most capture–recapture data and a multiple likelihood method for assigning recaptures to their most probable number of moults during time at large.