Research Document 2019/061
Identification of Provisional Reference Points and Harvest Rate Options for the Commercial Red Sea Urchin (Mesocentrotus franciscanus) Fishery in British Columbia.
By Lochead, J., Zhang, Z., and Leus, D.
Abstract
Length-based population models and projection simulations were used to study the impacts of a range of alternative harvest rates on British Columbia (BC) Red Sea Urchin (RSU) (Mesocentrotus franciscanus) populations. These models were based on a large accumulation of fishery-independent survey data and published estimates of growth, natural mortality and adult RSU spine canopy protection of juveniles (Zhang et al. 2008, 2011). Data were analyzed and results presented for the following regions of the coast: Haida Gwaii (Pacific Fishery Management Areas (PFMAs) 1, 2, 101 and 102), the Mainland North Coast (PFMAs 3 to 10 and 103 to 110), and the South Coast Inside Waters between Vancouver Island and the Mainland (PFMAs 11 to 20, 28, 29 and 111).
To evaluate the impact of a range of harvest rates, the concept of serious harm was considered in the identification of a provisional Limit Reference Point (LRP) and an Upper Stock Reference (USR). Reference Points were not previously identified for BC's RSU commercial fishery, and this work aligns this fishery with the Sustainable Fisheries Framework and DFO's Fishery Decision-Making Framework Incorporating the Precautionary Approach (DFO Precautionary Approach) (DFO 2009).
An empirical approach for establishing Reference Points based on historical density estimates from areas where Sea Otters (Enhydra lutris) had been established for at least five years is suggested. Our compilation of historical densities revealed RSU mean densities ranging from 0 to 4.01 RSU/m2, with a mean of 0.48 ± 0.20 (SE) and a median of 0.06 (Table 5) (Burt et al. 2018; Watson and Estes, 2011; Kvitek et al. 1989; Pearse and Hines 1987; Lowry and Pearse 1973; Faro 1970; Ebert 1968). From that range, we suggest 0.3 RSU/m2 from Watson and Estes (2011) as a reasonable LRP. This value was identified as the LRP because it was from BC in an area where Sea Otters had been established for at least 30 years, it was the longest time series of RSU density data in Sea Otter occupied areas (19 years) and it was the highest value of the Watson and Estes data (2011). Recognizing that recruitment events can cause large fluctuations in the number of small RSU observed during surveys, we propose the 0.3 RSU/m2 density threshold only applies to mature (≥ 50 mm TD) RSU, not RSU of all sizes. Therefore, we suggest the LRP is breached when the median of the bootstrapped sampling distribution of the mean spatial density on RSU habitat is less than 0.3 mature (≥ 50 mm TD) RSU/m2 and the USR is breached when the median of the bootstrapped sampling distribution of the mean spatial density on RSU habitat is less than 0.6 mature (≥ 50 mm TD) RSU/m2.
Decision Tables specifying the estimated probability of breaching the LRP and USR across a range of harvest rates are provided (Tables 12-23). Stock status was assessed at being above the USR at both the regional and coast wide spatial scales. The medians of the bootstrapped sampling distribution of the mean, mature RSU density, on all quadrats identified as RSU habitat, were estimated to be 2.51 RSU/m2 (95% CI: 2.11-3.25) for Haida Gwaii,1.70 RSU/m2 (95% CI: 1.57-1.95) for the Mainland North Coast, 0.89 RSU/m2 (95% CI: 0.81-1.06) for the South Coast Inside Waters, and 1.44 RSU/m2 (95% CI: 1.37-1.61) for all three regions combined.
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