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Innovative development of the Octopus (cf) tetricus fishery in Innovative development of the Octopus (cf) tetricus fishery in Western Australia

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The target species of the DOF is Octopus (cf) tetricus, an endemic species of Australia's southwestern temperate waters, ranging from Shark Bay in the north to the South Australian border in the south-east (Figure 5). Conversely, changes in management arrangements for the WCRLF have profoundly altered the dynamics of the fishing fleet.

Figure 1   a) octopus shelter pots, b) a schematic of shelter pots fixed to and being hauled  on a demersal longline
Figure 1 a) octopus shelter pots, b) a schematic of shelter pots fixed to and being hauled on a demersal longline

Age determination

  • Background
  • Age validation
  • Sample collection
  • Stylet increment analysis

For each image, five non-consecutive counts, using a hand counter, were made from the chemical mark to the edge of the style section. Paired t-tests were applied to determine if there was a significant difference between Reader 1 and each of the two independent researchers.

Figure 9   Location of a stylet in the mantle musculature of an octopus
Figure 9 Location of a stylet in the mantle musculature of an octopus

Life history

Background

Sample collection and maturation

Data analysis

Gear efficiency

  • Background
  • Fishing efficiency of gear
  • Standardised catch rate analysis
  • Leslie-Delury depletion experiment

This analysis was conducted for trigger traps alone due to their position as the dominant gear used in the expansion of the fishery. To estimate the abundance in the M1 and M2 nets and the catchability coefficient (q), the Leslie and DeLury depletion methods were used (Leslie and Davis, 1939; DeLury 1947).

Figure 14   Gonads of Octopus  (cf)  tetricus  (a) immature female, (b) immature male, (c)  maturing female and (d) maturing male  (from Larsen 2008)
Figure 14 Gonads of Octopus (cf) tetricus (a) immature female, (b) immature male, (c) maturing female and (d) maturing male (from Larsen 2008)

Biomass and sustainable harvest

  • Background
  • Estimating harvestable area and stock biomass
  • Per-recruit analysis
  • Empirical modelling of the effects of environment on potential harvest

For the DOF, the current focus area for fisheries expansion is the west coast. The width of the fishing area was then estimated by dividing the distance by the number of lures retrieved. Based on an average soaking time of 11 days per pot, a capture coefficient (q) of 0.117 was derived from the results of the depletion experiment (see Results).

The Hendrickson and Hart (2006) model was fitted to DOF-sampled data, which was reformulated to estimate the total mortality of the prespawn and egg components, denoted as Zn and Zs, respectively. Equal weights were applied to the binomial and multinomial likelihoods of the log and prior components of the composite objective function. The stored posterior values ​​of the estimated parameters were also used to calculate the posterior distributions of the fitted values ​​and thereby obtain the 95% confidence envelopes from the 2.5th and 97.5th percentile of these calculated values.

The model formulas for calculating N t( ) and S t( ) were summed to calculate the undistributed population abundance (ie, A t( )=N t( )+S t( )) for each of the following values ​​of . Z t( ) values ​​were calculated by subtracting from one the ratio of change in unassigned abundance from one month to the next.

Figure 18   Map of the octopus  fishing zones (by 1 degree Latitude) of the west coast of  Western Australia
Figure 18 Map of the octopus fishing zones (by 1 degree Latitude) of the west coast of Western Australia

Octopus predation on lobsters

  • Background
  • Predation analysis
  • Stylet increment analysis
  • Application of age data

The west coast's north-south orientation (zones A, B and C), relative uniformity of oceanic processes (minimal river outflow and upwelling), bathymetry (consistent shelf area), and coastal topography (few mountains or peninsulas), enabled latitude to be used as an environmental proxy for sea surface temperature ( SST). Of the three validation methods tested (OTC, AC and calcine), only the calcine treatment was successful, with 12 of the 15 injected squid showing calcine marks. These females either laid eggs or were about to lay eggs before the end of the experiment.

For each female, the calcine mark is stained at the style tip/lip border, suggesting that for females, stylet growth formation may cease at or shortly before oviposition. Without incontrovertible evidence to the contrary, the current study assumed that before breeding, females form daily growth rings throughout their lives in a similar way to males. All unsuccessful injections were the result of handling error, where due to insufficient needle penetration, calcine remained in the octopus flesh and was not captured by the stylets.

However, this may be a product of the small number of females sampled and gear selectivity or the potential effects of females ceasing to form surges during spawning. In (a) data divided into all data (white dot) and data that met the 5% precision rule (see section 1.4 for a description of the 5% precision rule).

Figure 21   Mean stylet increment count (±SE) subsequent to calcine injection as a function of  days post-injection
Figure 21 Mean stylet increment count (±SE) subsequent to calcine injection as a function of days post-injection

Life history

Maximum weight for females captured in trigger traps was 4400 g compared to 2700 g for males (Table 4). Females with ovaries weighing >80 g were considered highly gravid, with only one found in shelter pot samples and 161 found in trigger traps. All 22 spent females were captured in traps, 17 of which came from a single vessel, during a single day fishing at 40 m depth.

The reproductive rate for each of the 17 females was checked during processing and the abnormality of the event was noted. A large proportion of the total weight of highly pregnant females was attributed to reproductive organs, with gonads typically exceeding 250 g to a maximum of 482 g and a GSI of 13.5 (mean GSI = 4.5). The age profile of shelter pots and trigger traps was very similar for females and males.

For both sexes, traps mainly caught octopuses <350 days old, while trigger traps caught octopuses >250 days old. Trigger traps caught a mix of immature and adult females and almost no immature males (Figure 34a & b).

Table 4   Age and growth for shelter pot and trigger traps from biological sampling (standard  error)
Table 4 Age and growth for shelter pot and trigger traps from biological sampling (standard error)

Gear efficiency

Comparison of gear types

Standardised catch rate analysis

Leslie-DeLury depletion study

Biomass and Sustainable Harvest

  • Estimating stock biomass and harvestable area
  • Per-recruit model
  • Sustainable harvest
  • Empirical modelling of the effects of environment on potential harvest

Estimates of SPR (i.e., the ratio of SBPR at 2011 and 2012 fishing effort levels to unfished SBPR) were found to consistently decrease with decreasing age at sharp selectivity (tf) and Mn (Figure 47 a and b). Although the uncertainty of the estimated SPR turned out to be relatively wide, changes in the point estimate for SPR did not fall below SPR0.3 for the plausible ranges of input values ​​for tf and Mn and only fell below SPR0.4 relatively low values ​​were used when imports (i.e. the probability of growth overfishing, as indicated by lowering F0.1 to fall within the 95% credibility interval for Fˆ2011,2012, was demonstrated when tf fell below 6 months and when imported Mn fell below 0 .18 (Figure 47 c,d).

This reflects the change in the shape of the YPR curve from that shown in Figure 45, as the Mn input increased above 0.2, corresponding to a predicted downward slope of the YPR curve, which tends towards an asymptotic and infinite maximum at values unrealistically high. The vertical line shows the Bayesian posterior mean of fishing mortality from fitting the full model to data sampled in 2011 and 2012 combined, as a monthly F, with the shaded area encompassing the 95% confidence interval for this estimate. The range of the sustainable catch limit for the preliminary, conservative and potential scenarios showed that the actual catch of 240 t is much less than the lowest estimate for the preliminary scenario at SPR0.4 = 879 t.

Ranges are based on male and female weight-for-age for the lower and upper estimates, respectively. A strong negative correlation of -0.94 was evident between mean octopus size and mean SST by latitude, this was a very linear relationship (R2 = 0.88) indicating the relevance of using latitude as a proxy for temperature (Figure 49) . cf) tetricus by latitude for the west coast of Western Australia.

Table 5   Biomass and abundance estimates across each fishing zone for three scenarios  (Precautionary, Conservative and Possible)
Table 5 Biomass and abundance estimates across each fishing zone for three scenarios (Precautionary, Conservative and Possible)

Octopus predation on lobsters

An effective application of SIA requires care to ensure consistency in the preparation of the sections and the completion of counts. The age at the formation of the first increment (birth ring) for O. cf) tetricus or any other merobenthic squid species is currently unknown. Therefore, to ensure consistency across analysis, the present study maintains that the formation of the first stylet accretion is at hatching.

Since 2010, the strong differentiation in size and age composition of octopuses caught with the two fishing gears has played a decisive role in the development of the fishery. low catch rates, local effort and the transition from fishing to trapping. This differentiation in the efficiency of the two gear types is a product of catch composition and the shorter soaking period of trigger traps, and the reason behind their immediate impact on DOF.

In addition, the trigger traps had selectivity towards large, mature and older males. Information from these sources will assist in the annual assessment of fisheries and the monthly monitoring of catch rates for each zone.

Figure 50   Predation rates of octopus in lobster pots. O1 = octopus per 1000 lobster pot hauls  (black line), 02 = evidence of octopus predation per 1000 lobster pot hauls (grey broken  line) and number of lobster caught per 1000 lobster pot hauls (smooth
Figure 50 Predation rates of octopus in lobster pots. O1 = octopus per 1000 lobster pot hauls (black line), 02 = evidence of octopus predation per 1000 lobster pot hauls (grey broken line) and number of lobster caught per 1000 lobster pot hauls (smooth

Further development

To maintain the high level of spatial and temporal resolution of catch and effort data, the daily logbook recording system should be continued. In addition, all other reporting requirements should be streamlined to minimize possible double handling of data. Dedicated and consistent biological sampling from all fishing zones is required to maintain the integrity of the time series.

The research team has worked closely with the management team of the Fisheries Department on all aspects of the fishery. This report has been used by the Department of Fisheries, Western Australia to develop a management plan for the Octopus Interim Managed Fishery in Western Australia. Key recommendations on zone delineation, gear efficiency and sustainable harvesting levels have been used to set the allocation and access rules for the fishery.

As part of the wider scientific community, the research team maintained and strengthened links with a variety of universities, research institutes and government agencies across Australia, including: The University of Tasmania, Murdoch University, Museum Victoria, La Trobe University, New Castle University, Australia Museum , Museum of Western Australia, Museum Victoria, NSW Fisheries, South Australian Research and Development Institute, Department of Primary Industries Water and Environment (Tas.) and Fisheries Victoria. These relationships were of mutual benefit and covered a variety of biological, ecological and fisheries management issues.

Project coverage

Beak raising counts as a tool for growth studies of the common octopus Octopus vulgaris in southern Brazil. Behavior of the Western Australian spiny lobster, Panulirus cygnus Geroge, in the field and in the laboratory. Western Australia's fisheries and aquatic resources status reports 2012/13: the state of the fisheries.

The importance of retention processes in emerging areas for the recruitment of Octopus vulgaris: the example of Arguin Bank (Mauritania). Western Australia's Fisheries and Aquatic Resources Status Reports 2012/13: State of the Fisheries. Western Australia Fisheries and Aquatic Resources Status Reports 2012/13: State of the Fisheries.

Reproductive biology of the common octopus, Octopus vulgaris Cuvier, 1797 (Cepahlopoda: octopodidae) in the Gulf of Cádiz (SW Spain). Description of the clay pot fishery in the Gulf of Cadiz (SW Spain) for Octopus vulgaris: Selectivity and pattern of exploitation.

Figure

Figure 1   a) octopus shelter pots, b) a schematic of shelter pots fixed to and being hauled  on a demersal longline
Figure 5   Range  of  distribution for Octopus  (cf)  tetricus  and  current partitioning of the  fishery.
Figure 6   Octopus (cf) tetricus
Figure 9   Location of a stylet in the mantle musculature of an octopus
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References

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