Comparative Habitat Use of Estuarine Habitats with and Without Cultch-on-Longline Gear Present

Few studies have addressed how and whether off-bottom oyster culture affects species use and abundance, or whether these differences affect the overall food web ecology of the system. Confluence designed and is leading a grant-funded study to research comparative habitat use of estuarine habitats with and without cultch-on-longline gear present. The project addresses the goal of providing research on the environmental impacts of shellfish aquaculture by furthering the understanding of how fish and invertebrate communities are affected by the presence of cultch-on-longline oyster aquaculture. The project also explores the utility of Ecopath with Ecosim (EwE) modeling software to evaluate the effect that oyster culture has on the food web and commercial fisheries.

Approach and Findings

Shellfish aquaculture, native eelgrass, and mudflat habitat have co-existed in Humboldt Bay for at least the last 60 years of commercial shellfish production, and for more than 120 years since the first attempts to introduce cultured shellfish in 1896. This project addresses questions about the environmental impacts of shellfish production using off-bottom oyster aquaculture growing techniques. Specifically, this study compares fish and invertebrate populations at sites where oysters are grown commercially to similar habitats that do not have oyster aquaculture present. This study focused on 3 resources, asking “Does oyster culture alter (1) fish assemblages, (2) invertebrate assemblages (prey resources), and (3) the food web?

Fish: By building an enclosure net, research scientists measured fish populations at 4 locations: (1) with oyster culture over eelgrass, (2) without oyster culture over eelgrass, (3) with oyster culture over mudflat, and (4) without oyster culture over mudflat. While the timing and abundance of fishery species varies seasonally with some species and life stages occurring in Humboldt Bay for only short periods of time, the study found no significant difference between species abundance and assemblages between sites based on the presence of aquaculture gear. Fish are more abundant, and populations are more diverse in eelgrass habitats compared to mudflat habitats. Results were compared to earlier work by William Pinnix (U.S. Fish and Wildlife Service Supervisory Fish Biologist) and sampling by Brett Dumbauld (U.S. Department of Agriculture Research Scientist) in Humboldt Bay. These additional observations are consistent with the findings that oyster aquaculture does not appear to significantly affect fish population abundance or diversity. However, the underlying habitat type and specifically, the presence of eelgrass is associated with higher abundance of fish.

There are some underlying trends that would require additional sampling effort to verify. Sampling suggests that some larval or early juvenile fish species may be associated with oyster culture gear. These include juvenile herring and topsmelt. It is possible that the presence of culture gear in the water column provides these fish with either additional foraging or refuge habitat. These life stages are only present in the nearshore for relatively short periods of time so additional sampling during the late Spring time period would be needed to verify these tentative observations.

Invertebrate assemblages: Invertebrate samples were collected throughout Humboldt Bay during a winter and a summer sampling period to identify potential differences in invertebrate abundance or species. While invertebrates are more diverse and abundant in the summer sampling period, samples inside and outside of aquaculture suggest that aquaculture is not having a significant affect on invertebrate populations in Humboldt Bay.

Food Web: To understand how oyster culture may interact with the food web in Humboldt Bay, the project team hosted a symposium to gather input from current and past researchers to better understand how the food web in Humboldt Bay is structured and what data sets are available to support analysis of potential interactions. While there are several potential interactions, as field study portions of this study indicate, there is limited evidence that oyster aquaculture has significant impacts to the food web. Studies are ongoing to understand the interactions between aquaculture and eelgrass, as well as to understand whether the presence of aquaculture gear affects bird use of habitats.

Project Location
Humboldt Bay, California

U.S. Department of Commerce: Saltonstall-Kennedy NOAA Fisheries Competitive Research Grant

Project Timeline
2016 – 2019

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