Previous Reference Points Reference Points and Assessment Models

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6. Reference Points and Assessment Models

6.1. Previous Reference Points

In all previous assessments reference points were estimated independently of the analyses used to assess population abundance and exploitation. We summarize the approaches below. 6.1.1. BBCAC Reference Points The BBCAC TSC recommended overfished and overfishing definitions and a target exploitation rate Miller 2001b. The overfished definition was based on the average abundance of age‐1+ crabs in the four principal fishery‐independent surveys see Section 3.2. Survey Z‐scores from each survey were averaged to yield a single abundance measure for age‐1+ crabs. The threshold reference point was chosen as the lowest survey Z‐score in the time series – the 1968 abundance. This recommendation was based on purely empirical reasoning that abundances lower than this level could not be shown to have supported a sustainable fishery. Both exploitation rate reference points were developed from a traditional Beverton‐Holt yield per recruit analysis. The overfishing definition was selected as the exploitation rate that maintained 10 of the spawning potential F 10 and the target as that level that maintained 20 of the spawning potential F 20 . These reference points were used for management from 2001 ‐ 2006. 6.1.2. Individual‐based Per Recruit Reference Points New reference points were adopted for management of the blue crab population in Chesapeake Bay based on analyses carried out for the 2005 assessment Bunnell and Miller 2005. As with the BBCAC reference points, these newer reference points were estimated independently of the analyses to assess stock status. The adopted approach used an individual‐based simulation model to track the yield of a hypothetical cohort of 300 million crabs over three years. During the simulation crabs grew according to a temperature‐dependent molt‐process model Brylawski and Miller 2006. Reproduction was estimated using published estimates of maturity Sharov et al. 2003, fecundity Prager et al. 1990 and brood production Hines et al. 2003. Crabs died in the model due to either natural mortality or fishing. The model was used to forecast spawning potential per recruit isoclines as a function of natural mortality and fishing mortality. In a change from the BBCAC reference points, exploitation was represented as an exploitation fraction i.e. catch initial abundance rather than as the instantaneous rate. However, Bunnell and Miller still recommended use of the 10 and 20 SPR levels as reference points. 39 These reference points have been the foundation for management decisions since 2006. 6.1.3. CBSAC Interim Target Stakeholders and managers began expressing concerns over the exploitation‐ based management strategy soon after the 2005 assessment. Managers increasingly relied on the abundance estimate from the winter dredge survey as the primary indicator of stock status. Thus, managers were concerned that focusing on an exploitation rate strategy removed attention from the efforts to sustain the crab population at desirable levels of abundance. Accordingly, CBSAC recommended an interim abundance target of 200 million age‐1+ crabs baywide. This figure was based on analyses of the relationship between winter dredge‐based estimates of abundance and harvest, and abundance and recruitment Chesapeake Bay Stock Assessment Committee 2008. Thus from 2008 onwards, the management control rule involved an empirical overfished definition, spawning potential per recruit exploitation fraction limits and targets and an interim abundance target. The current control rule for the Chesapeake Bay blue crab fishery is depicted in Figure 6.1. Shown on this figure are the four key management reference points Limits Overfishing definition: exploitation rate, U=0.53 Overfished definition: 86 million age 1+ crabs Targets Exploitation target: Exploitation rate, U=0.46 Interim abundance target: 200 million age‐1+ crabs Estimation of the status of the crab population and its fisheries in each year rely on reported catches in the fishing year together with empirical estimates from the winter dredge survey in the preceding winter Sections 5.4.3 – 5.4.4. Data presented in Figure 6.1 indicate that the blue crab population was above the interim abundance target, and the fishery was operating below the target exploitation rate in 1990, the first year in which data from the winter dredge survey were available. Exploitation rates increased and abundances declined, such that by 1995 the crab population was below the interim abundance target and the fishery was operating above the overfishing definition. This situation continued and in fact worsened for the next five years, such that by 1999, the fishery was removing almost 80 of the available crabs. Subsequent management actions effectively reduced exploitation fractions, but failed to lead to significant increases in population abundance. For example in 2008, the exploitation fraction U=0.49 had declined to below the overfishing threshold. This represented a 37 decline in exploitation rates from its 1999 peak. However, the population 40 abundance in 2008 had only increased by 48 and was still substantially below the interim target. Subsequently, the effect of the female conservation measures on abundance have been substantial. Exploitation rates have changed on only modestly U 2009 =0.44, but abundances in 2009 almost doubled from their 2008 values N 2009 =235.1 million. Although the current reference points and approach to management appear to be working, in making its 2008 recommendation, CBSAC noted that the interim target was not fully integrated in the existing reference point framework. Specifically it was noted that it was possible that attaining the exploitation rate target of U 20 and the interim abundance target of 200 million crabs may be mutually exclusive. Thus CBSAC recommended that a principal goal of any subsequent assessment was to bring forward new reference points from an integrated analysis that simultaneously estimates reference points and stock status.

6.2. Sex‐specific catch, multiple survey model