Historical ecology and the conservation of large, hermaphroditic fishes in Pacific Coast kelp forest ecosystems

Abstract

The intensive commercial exploitation of California sheephead (Semicossyphus pulcher) has become a complex, multimillion-dollar industry. The fishery is of concern because of high harvest levels and potential indirect impacts of sheephead removals on the structure and function of kelp forest ecosystems. California sheephead are protogynous hermaphrodites that, as predators of sea urchins and other invertebrates, are critical components of kelp forest ecosystems in the northeast Pacific. Overfishing can trigger trophic cascades and widespread ecological dysfunction when other urchin predators are also lost from the system. Little is known about the ecology and abundance of sheephead before commercial exploitation. Lack of a historical perspective creates a gap for evaluating fisheries management measures and marine reserves that seek to rebuild sheephead populations to historical baseline conditions. We use population abundance and size structure data from the zooarchaeological record, in concert with isotopic data, to evaluate the long-term health and viability of sheephead fisheries in southern California. Our results indicate that the importance of sheephead to the diet of native Chumash people varied spatially across the Channel Islands, reflecting modern biogeographic patterns. Comparing ancient (~10,000 calibrated years before the present to 1825 CE) and modern samples, we observed variability and significant declines in the relative abundance of sheephead, reductions in size frequency distributions, and shifts in the dietary niche between ancient and modern collections. These results highlight how size-selective fishing can alter the ecological role of key predators and how zooarchaeological data can inform fisheries management by establishing historical baselines that aid future conservation.

Keywords: Channel Islands; Dietary Niche; Restoration Ecology; Zooarchaeology.

Fig. 1. Location of northern Channel Island archaeological sites with sheephead remains identified to species.

Sea surface temperature (SST) gradient represents mean SST for a 10-year period from June 2006 to June 2016 (93). Relative abundances of sheephead are reported as a percentage of the total fish NISP (base map courtesy of L. Reeder-Myers). Note that site locations are approximate due to confidentiality concerns of Channel Island National Park.

Fig. 2. Change in relative abundance of sheephead remains in archaeological sites on the northern Channel Islands through time.

Sheephead abundance is represented by average %NISP with SE per time period from Table 2. Inset: Image of a male California sheephead likely captured from southern California in ca. 1910 CE (open access image via Wikimedia Commons).

Fig. 3. Change in mean total length (TL) of sheephead through time.

Dots represent means, and error bars represent SDs. Inset: Image of a California sheephead pharyngeal showing the measurement used to obtain these data. Data are derived from Supplement 2 (table S1).

Fig. 4. California sheephead size through time along the northern Channel Islands.

(A) Size distribution of California sheephead from the northern Channel Islands. Black bars are from archaeological samples, and gray bars represent modern sizes collected during 2007 and 2008 (for data, see Supplements 2 and 3). (B) Size spectra analysis using the estimated weights of sheephead from archaeological and modern samples.

Fig. 5. Dietary shifts in California sheephead through time across the northern Channel Islands.

Carbon and nitrogen isotopic compositions for archaeological sheephead bone collagen from ANI-2 (filled red circles) and SMI-232 (filled blue circles), modern sheephead bone collagen from Anacapa Island (open blue triangles), and historic sheephead bone collagen from San Miguel Island (open red triangles) (for data, see Supplement 4). The shaded ellipses represent the standard bivariate ellipse areas for each of the four groups.