Figure 1: The decision tree of the trend analysis, showing where each Research Block belongs this year.

Figure 1: The decision tree of the trend analysis, showing where each Research Block belongs this year.

Background

For data-limited toothfish fisheries in the Southern Ocean there are generally no fishery-independent data on the status of the stock. Therefore, the collection of such data is included in the CCAMLR management process that sets the requirements for vessels to participate in those fisheries.

Determining the appropriate catch limits in data-limited toothfish fisheries that allow sufficient data collection for stock assessments, but that do not place stocks at risk in the interim of having an assessment, is a widespread topic in fisheries generally and one that has been a longstanding issue for CCAMLR (SC-CAMLR-XXIX, paragraphs 3.128-3.129).

In 2016, CCAMLR agreed to an approach for estimating biomass in data-limited areas that uses two methods, the CPUE-by-seabed area analogy and the Chapman mark-recapture estimation (WG-SAM-16 paragraph 2.28).

In theory the two methods for biomass estimation should provide similar results, however, differences in estimates between methods may arise due to limitations in these methods (e.g., these do not account for varying tag survival or fish migrations).

In 2017, CCAMLR (WG-FSA-17 paragraphs to 4.28 to 4.38) agreed that, where time-series of such biomass estimates are available, the trends in these estimates, as well as the most recent estimates of biomass, should be used to provide information on setting and/or changing catch limits.

CPUE-by-seabed area analogy

A central element that underpins any assessment is the estimation of the biomass of the target species in the proposed fishing area. While CCAMLR has a number of data-limited toothfish fisheries it also has data-rich fisheries that are assessed using an integrated assessment software (CASAL), and, information from these assessed fisheries is used to guide the assessment of data-limited fisheries.

For data-limited fisheries, an initial estimate of biomass can be obtained using the biomass estimated in a data-rich area and assuming the same density of fish in the data-limited area. As fishing data becomes available this simple ‘seabed area’ approach scales biomass by the ratio of the Catch Per Unit Effort (CPUE; kg of fish caught per km of fishing line) in the data-rich area to that in the data-limited area (i.e., the CPUE-by-seabed area analogy method; Agnew et al., 2009) such that the biomass B can be estimated as:

Where the subscripts x and r denote parameters from the research block and reference/assessed area respectively. C is the median of the haul by haul CPUE where the total catch (t) on a line, including fish that are tagged and released, is divided by the length of line (km). A is the seabed area (km2) in the depth range 600-1800m and \(B_r\) is the current biomass estimate (t) from the most recent assessment in the reference area.

Chapman mark-recapture estimation

A requirement for participating in all CCAMLR data-limited toothfish fisheries is to tag and release fish at a rate of 3 to 5 fish per tonne caught such that mark-recapture data can be used to estimate biomass (e.g., via the Peterson/Chapman estimation methods).

The Chapman mark-recapture-based biomass estimation relies on the assumption that the ratio of tagged and un-tagged fish in the population is represented by that ratio in the catch. The biomass B in fishing season s can be estimated as:

where \(n_{s-1}\) is the number of tagged fish available for recapture at the end of the previous fishing season, \(C_s\) is the catch in season s (as with CPUE the catch includes fish that are tagged and released) and \(m_s\) is the number of tagged fish recaptured in season s (excluding within-season recaptures).

Trend Analysis decision rules

A set of trend analysis decision rules were developed by the Working Group on Fish Stock Assessments (WG-FSA-17) to determine a catch limit for each research block in a data-limited fishery. These decision rules have evolved over time to adapt management to fisheries dynamics.

In recent years, due to the lack of consensus over fishing in some areas (e.g., CCAMLR-40 paragraph 6.43), WG-SAM-21 (paragraph 3.32 (iii)) agreed that in order to establish catch limits in research blocks where fishing had not taken place in recent fishing seasons, if data were not available from the most recent fishing season, the previous catch limit should be carried forward. Such an approach should be limited to five years, after which time the catch limit would need to be re-evaluated outside the current trend analysis framework (see Fig. 1).

Within the Chapman method, due to the potential bias introduced when few fish are recaptured, trends are considered only where there were sufficient recaptures, which is defined as being at least three recaptures per year in at least two of the last three years (WG-FSA-17 paragraph 4.33).

The time-series of biomass estimates from both methods are used to evaluate overall trends in biomass. An inverse variance weighted least-squares regression is used to incorporate the confidence of each biomass estimate in the determination of the trend in the biomass time-series. In order to compare between trends across research blocks the standardized regression (beta) coefficient of the slope is estimated.

Each trend is then evaluated using a threshold of beta=0.1 so that the overall trend is determined to be:

In cases where there is not sufficient data to compute a trend in Chapman estimates, the overall trend would be determined to be Unclear.

Following the trend evaluation, a decision tree (Fig. 1) is used to determine the appropriate biomass estimate and associated catch limit at an exploitation rate of 4% of that biomass estimate. This process further includes a maximum change of +/- 20% in the catch limit between years in order to provide stability in the planning process.

The latest estimates obtained using this method are shown in Table 1 and a map of those research blocks is shown in Figure 2. Recommended catch limits are subject to approval by the Commission.

Table 1. Latest Research Blocks biomasses (B, tonnes) and catch limits (CL, tonnes) estimated using the trend analysis. PCL: previous catch limit; ISU: increasing, stable or unclear; D: declining; Y: Yes; N: No; -: No fishing occurred in the last Season. Recommended catch limits are subject to approval by the Commission.
Subarea or Division Research Block Species PCL Trend decision Adequate recaptures CPUE Trend Decline B Bx0.04 PCLx0.8 PCLx1.2 Recommended CL for 2022
48.6 486_2 D. mawsoni 112 ISU Y N 5617 225 90 134 134
486_3 D. mawsoni 30 ISU N N 957 38 24 36 36
486_4 D. mawsoni 163 ISU Y Y 10816 433 130 196 196
486_5 D. mawsoni 263 D Y Y 15036 601 210 316 210
58.4.1 5841_1 D. mawsoni 138
138
5841_2 D. mawsoni 139
139
5841_3 D. mawsoni 119
119
5841_4 D. mawsoni 23
23
5841_5 D. mawsoni 60
60
5841_6 D. mawsoni 104
104
58.4.2 5842_1 D. mawsoni 60 ISU Y N 3416 137 48 72 72
58.4.3 5843a_1 D. eleginoides 19
19
58.4.4 5844b_1 D. eleginoides 18
18
5844b_2 D. eleginoides 14
14
88.2 882_1 D. mawsoni 192 ISU Y N 6588 264 154 230 230
882_2 D. mawsoni 186 ISU Y Y 17892 716 149 223 223
882_3 D. mawsoni 170 ISU N N 5308 212 136 204 204
882_4 D. mawsoni 128 ISU Y Y 8274 331 102 154 154
882H D. mawsoni 128 D Y Y 4500 180 102 154 102
88.3 883_1 D. mawsoni 16
16
883_2 D. mawsoni 20
20
883_3 D. mawsoni 60
60
883_4 D. mawsoni 60
60
883_5 D. mawsoni 8
8
Figure 2: Location of the Reference Areas (orange) and Research Blocks (red) for which the trend analysis was last used. The fishable depth range (600m-1800m) is highlighted in shades of green.

Figure 2: Location of the Reference Areas (orange) and Research Blocks (red) for which the trend analysis was last used. The fishable depth range (600m-1800m) is highlighted in shades of green.

Additional Resources