Monday August 29, 2022

The establishment of California’s Marine Protected Areas (MPAs) in 2012 was a crucial step for conserving and protecting vital habitats, not only for marine, but also freshwater species. MPAs are areas with permanent legal protections that aim to conserve critical natural resources. The restrictions within MPA’s vary, but include a limit or ban on the harvest or take of some or all resources. MPAs exist within estuaries and marine areas that have been identified as essential habitat for a variety of wildlife, including salmon and steelhead. Salmon and steelhead depend heavily on estuaries as juveniles, where they rear in the food-rich and protected estuary environment. By lingering in estuaries, these two critical fish species increase their chances of survival upon entering the ocean. Although MPA designation plays a major role in salmon and steelhead success, the majority of MPA protections focus on larger-sized estuaries, such as San Francisco and Morro bays. These larger estuaries have more stakeholders and, therefore, more focus, attention, and funding when compared to smaller estuaries. However, many of the coastal estuaries in Northern California, are small and understudied, and at great risk due to high rates of sedimentation from upstream timber harvesting. A study on Northern California estuaries by masters student Katherine Osborn at Cal Poly Humboldt demonstrated that, despite their size, smaller estuaries provide critical habitat to a large number of fish and invertebrate species. 

In this study, researchers sampled three different Northern California estuaries to provide a baseline of the seasonal fluctuations within fish communities. At the beginning of the study, two of the estuaries – Big River estuary MPA and Ten Mile estuary – had recently been designated as MPAs, and the third estuary – Mad River – was unprotected. However, the researchers did not expect to see any significant impacts from these protection effects given the recency of the MPA designations. Each estuary was sampled biannually, once in summer and once in winter. At each of the three study sites an upstream location and a downstream location closer to the mouth of the estuary were chosen for sampling. The fish community was sampled with a combination of beach seines and fyke traps. Benthic invertebrates were also collected to establish abundance and diversity estimates. In order to understand seasonal community shifts as well as their upstream extent, the researchers performed additional sampling in the Mad River estuary that included a third sampling location farther upstream.

Across all three study sites, the researchers found that fish community trends were strongly driven by season. The fish catch in summer was abundant and diverse, and included periodic estuarine users like salmon and steelhead. Meanwhile, winter catches were small and consisted mostly of estuarine residents, such as sculpin. The researchers also found that fish communities across all three sites were more abundant and diverse in downstream areas compared to sites further upstream. The Big River estuary had the strongest ocean connection and, therefore, the most abundant and diverse fish community. In contrast, the Ten Mile River estuary had the weakest ocean connection, as the mouth sometimes closes and becomes a freshwater lagoon. Freshwater lagoons are also beneficial to outmigrating salmon as it gives them a chance to overwinter and become stronger before heading out into the harsh ocean environment. The increased sampling effort at the Mad River estuary did not yield any additional species. However, it did show that fish abundances increased from the spring through the summer, and that higher ocean connectivity led to a larger and more diverse presence of fish and invertebrate species. The catches at Mad River estuary were mostly dominated by fish that resided in the estuary year-round. Overall, the researchers found that season was the most important factor in determining the composition of species, and distance from the ocean was most important in determining the number of species present. Invertebrate diversity, on the other hand, was shown to be less affected by seasonality and more affected by differences among the estuaries themselves. 

The study concluded that despite their more diminutive size, these small estuaries exhibited similar characteristics to larger estuaries in terms of species diversity, highlighting the critical importance of protecting all different sizes of estuaries that harbor species of conservation concern. Small estuaries are often overlooked in conservation efforts, but it has become increasingly clear to scientists that the effects of habitat degradation and climate change do not discriminate. Ensuring the protection of these smaller estuaries through a network of MPAs may help protect these vital habitats for the fishes and wildlife that rely on them.

This post was featured in our weekly e-newsletter, the Fish Report. You can subscribe to the Fish Report here.

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