Effects of experimental harvest on red sea urchins (Strongylocentrotus franciscanus) in northern Washington.

Abstract--Commercial harvest of red sea urchins began in Washington state in 1971. Harvests peaked in the late 1980s and have since declined substantially in Washington and other areas of the U.S. west coast. We studied effects of experimental harvest on red sea urchins in San Juan Channel (SJC), a marine reserve in northern Washington. We recorded changes in density and size distribution of sea urchin populations resulting from three levels of experimental harvest: 1) annual size-selective harvest (simulating current commercial urchin harvest regulations), 2) monthly complete (non-size-selective) harvest, and 3) no harvest (control) sites. We also examined recolonization rates of harvested sites. The red sea urchin population in SJC is composed of an accumulation of large, old individuals. Juvenile urchins represent less than 1% of the population. Lower and upper size limits for commercial harvest protect 5% and 45% of the population, respectively. Complete harvest reduced sea urchin densities by 95%. Annual size-selective harvest significantly decreased sea urchin densities by 67% in the first year and by 47% in the second year. Two years of size-selective harvest significantly altered the size distribution of urchins, decreasing the density of legal-size urchins. Recolonization of harvested sites varied seasonally and occurred primarily through immigration of adults. Selective harvest sites were recolonized to 51% and 38% of original densities, respectively, six months after the first and second annual harvests. Yields declined substantially in the second year of size-selective harvest because of the fishing down of the population and because of low recolonization rates of harvested sites. We recommend that managers consider the potential efficacy of marine harvest refuges and reevaluate the existing upper and lower size limits for commercial harvest to improve long-term management of the sea urchin fishery in Washington.

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Red sea urchins (Strongylocentrotus franciscanus) are the most commonly harvested species of sea urchin on the west coast of North America. Sea urchin harvest in this region occurs primarily in California, Washington, and British Columbia. The commercial sea urchin fishery in Washington began in 1971, and landings were low through the early 1980s (Fig. 1). Landings increased dramatically in the late 1980s, peaking at over 4000 metric tons (t) in 1988. Landings have since declined. Approximately 387 t of sea urchins were harvested in 2000, valued at $699,052 (Ulrich (1)). Red sea urchins currently constitute approximately 60% of landings--the remainder being green sea urchins (S. droebachiensis, Ulrich (1)).

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The Washington sea urchin fishery currently is managed by using harvest quotas, size limits, license restrictions, limited entry, and mandatory log books (Lai and Bradbury, 1998). Season length is not limited, but harvest occurs primarily during the winter when roe quality is highest (Bradbury (2)). A rotational harvest strategy was practiced from 1977 until 1995, in which each harvest district (Fig. 2) was harvested once every third year. In 1995 a U.S. federal court ruling on shellfishery management in Washington's coastal marine waters (Shellfish Subproceedings of United States vs. State of Washington, 873 F. suppl. 1422, 1994, known commonly as the "Rafeedie Decision") allotted one half of all harvestable shellfish to native tribes. As a result, rotational harvest was discontinued and replaced by annual harvest to ensure that all tribes had equal access to their usual and accustomed fishing areas each year.

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Sea urchin harvests declined substantially in the early 1990s along parts of the west coast (Fig. 1). Quotas and season lengths were reduced in Washington because of overharvesting concerns (Bradbury(2)). Quotas were not reduced in California, and catches declined substantially (Kalvass and Hendrix, 1997). Sea urchin densities in some harvested areas in northern California are less than one quarter of those in nearby reserve areas (Kalvass and Hendrix, 1997). In Washington, densities and the proportion of legal-size sea urchins in the population have declined (Pfister and Bradbury, 1996). Harvesters may be maintaining catch per unit of effort at high levels by exploiting new populations, thereby masking stock declines (Pfister and Bradbury, 1996).

Experimental harvests may indicate potential effects of commercial harvest on sea urchin populations in Washington. We examined changes in density and size distribution of red sea urchin populations resulting from two levels of experimental harvest. Our study site was San Juan Channel (SJC), one of two areas in Washington where commercial sea urchin harvest has been prohibited since the 1970s (Fig. 2). We compared density and size distribution data for sea urchins from experimentally harvested sites in SJC with similar data collected from 1) nearby control sites, and 2) 19 sites in the Strait of Juan de Fuca (SJDF), an area commercially harvested since the early 1970s. We also examined recolonization of harvested sites through recruitment and immigration. We discuss implications of our results for management of sea urchin harvest effort and the potential use of marine harvest refuges in enhancing Washington's sea urchin fishery.

Methods

We established nine study sites in SJC from November 1996 to March 1997 (Fig. 2). Sites were 6-10 m in depth and approximately 10 m x 40 m, with the long axis of each site running parallel to the shoreline along the depth contour (Table 1). Eight permanently marked circular sampling areas (each 7.07 [m.sup.2]) were located along the midline of each site along the depth contour. Site selection was based on high red sea urchin density ([greater than or equal to] 1.5/[m.sup.2] in preliminary surveys), substrate (primarily large cobble or bedrock) and safety considerations.

We applied one of three harvest treatments to each site. "Selective harvest" consisted of annual removal of all legal-size sea urchins (102-140 mm test diameter) each winter (March of 1997 and 1998). Selective harvest simulated the annual commercial harvest of a bed of sea urchins in the San Juan Islands (Pfister and Bradbury, 1996). "Complete harvest" consisted of removal of all sea urchins present in March 1997, and at monthly intervals thereafter through September 1998. The complete harvest treatment did not represent any current management strategy for commercial harvest. Rather, the complete harvest treatment represented one extreme (the control treatment being the second) against which effects of selective harvest may be compared. Sea urchin densities were not manipulated in the "control" treatment. Because of logistical constraints, control sites were located in smaller preserve areas within SJC where harvest of all invertebrates and fish is prohibited. Harvest treatments were randomly assigned to the remaining six sites. All sea urchins harvested were measured at the surface and released at other locations in SJC well away from the study sites.

In SJC, we sampled at large (sites) and small (circular areas within sites) spatial scales. Small-scale sampling was more frequent, allowing detection of potential short-term effects of harvest; whereas large-scale sampling better indicated how treatments affected the entire bed of sea urchins. We identify the scale of sampling (sites vs. circular areas within sites) for all results.

Size distribution

In SJC, we measured all sea urchins removed during the initial harvest of complete and selective harvest sites in March 1997, and all sea urchins removed during the second annual harvest of selective harvest sites in March 1998. In September of 1997 and 1998, we measured a minimum of 100 sea urchins in situ in all sites in randomly chosen 5 m x 5 m quadrats. We also measured sea urchins in the circular areas within sampling sites in situ in all sites five times …