Quality of a fished resource: Assessing spatial and temporal dynamics

Abstract

Understanding spatio-temporal variability in the demography of harvested species is essential to improve sustainability, especially if there is large geographic variation in demography. Reproductive patterns commonly vary spatially, which is particularly important for management of “roe”-based fisheries, since profits depend on both the number and reproductive condition of individuals. The red sea urchin, Mesocentrotus franciscanus, is harvested in California for its roe (gonad), which is sold to domestic and international sushi markets. The primary driver of price within this multi-million-dollar industry is gonad quality. A relatively simple measure of the fraction of the body mass that is gonad, the gonadosomatic index (GSI), provides important insight into the ecological and environmental factors associated with variability in reproductive quality, and hence value within the industry. We identified the seasonality of the reproductive cycle and determined whether it varied within a heavily fished region. We found that fishermen were predictable both temporally and spatially in collecting urchins according to the reproductive dynamics of urchins. We demonstrated the use of red sea urchin GSI as a simple, quantitative tool to predict quality, effort, landings, price, and value of the fishery. We found that current management is not effectively realizing some objectives for the southern California fishery, since the reproductive cycle does not match the cycle in northern California, where these management guidelines were originally shaped. Although regulations may not be meeting initial management goals, the scheme may in fact provide conservation benefits by curtailing effort during part of the high-quality fishing season right before spawning.

Disentangling the effects of fishing and environmental forcing on demographic variation in an exploited species

“…wildlife.ca.gov]). In addition, higher GSI is an indicator of superior gonad quality, which translates to higher prices (Kalvass and Hendrix, 1997;Unuma, 2002;Teck et al, 2017). We found that red sea urchins were larger with higher quality gonads in the western region, even though these sites experience consistently higher commercial fishing pressure (Teck et al, 2017).…”

“…Evidence from this study can help to explain the presence of a strong gradient in fishing pressure across the Channel Islands, with significantly higher red sea urchin landings in the west versus the east (Shears et al, 2012;Teck et al, 2017). In addition, our results showing lower Lines show linear regressions across longitude and between MPA and unprotected areas (density: R 2 = 0.08, F 3,142 = 3.80, P = 0.0118; adult biomass density: R 2 = 0.09, F 3,142 = 4.33, P = 0.0060; reproductive biomass density: R 2 = 0.09, F 3,142 = 4.56, P = 0.0044).…”

“…several prominent regional differences in sea urchin demographics, which help explain the regional differences in fishing pressure and fishermen behavior across the northern Channel Islands (Shears et al, 2012). Despite higher fishing mortality rates in the west (Shears et al, 2012;Teck et al, 2017), red sea urchins were still larger and had greater reproductive potential in the western (colder) region. This result is partially due to the enhanced primary productivity and greater kelp resource availability in the western region (Shears et al, 2012;Appendix G), and for many species, including red sea urchins, more food availability can result in higher reproductive output (Claisse et al, 2013).…”

“…This site supports the highest densities of red urchins at any of the sites examined in this study (8 ± 2.3 adults m −2 in comparison to the average across all sites of 2 ± 0.2 adults m −2 ). The lower densities at nearby unprotected sites may reflect the heavy fishing pressure in this region (Teck et al, 2017) that alleviates resource competition for the remaining urchins.…”

“…We examined two individual-based metrics of adult red sea urchin: size (test diameter) and gonadosomatic index (GSI), which is a strong metric of reproductive stage and value to the fishery (Unuma, 2002;Teck et al, 2017):…”