Science Friday: Sand bars, lagoons and moving steelhead
Back to back Science Friday posts focusing on California and the unique challenges that steelhead face in the state. This week we pivot from juvenile steelhead coping with ponded pools to a look at juveniles that enter and live seasonally in lagoons at the mouths of creeks and rivers — a phenomenon fairly common in smaller coastal watersheds in California but less common in larger river systems and north of the Eel River.
Typically, after flows in such systems diminish in late spring or early summer, their connection to the ocean will be closed by a sandbar. For the next half-year or so, or until winter storms boost flows enough to breach that sandbar, juvenile O. mykiss will often utilize this estuarine habitat as upstream habitat dries back or is otherwise degraded by higher temperatures and reduced streamflow.
A recent study looked at juvenile steelhead in the lagoon of Scott Creek, a small but well-studied coastal stream a few miles north of Santa Cruz. The lagoon typically forms in midsummer, around late July, and persists until late fall or early winter (average date of reconnection is November 29) when storms produce sufficient streamflow to erode the sandbar and reconnect the creek to the ocean.
As any steelheader knows, a creek needs to connect to the ocean for smolts to out-migrate and for adults to return and spawn. In 2015, a year of extreme drought, the sandbar on Scott Creek formed over two months earlier than average and lasted until December 21. Steelhead smolt in the spring, which meant that in 2015 many smolts could not get to the ocean during that time. Under such conditions, their options are to remain in the lagoon until the bar is breached or move back upstream and rear in the creek.
Researchers took advantage of the extreme drought conditions to investigate how steelhead responded. The study was conducted by Ann-Marie Osterback and several co-authors (the paper can be found here).
The Osterback team sampled the lagoon twice per month over a seven-month period, relying on PIT tags to track growth and movement of individuals, the tags are about the size of a grain of rice and are installed in the body of the fish. At the end of the study they evaluated the abundance, growth, movement of the juvenile steelhead as well as their readiness for the marine environment.
The authors found juvenile steelhead, and coho salmon (which we don’t cover here), in the lagoon during every month of sampling. These fish persisted in the lagoon for extended periods of high water temperature and low dissolved oxygen levels. Abundance of juvenile steelhead peaked after formation of the lagoon at about 550 fish in June, but thereafter declined throughout the study period to about 350-380 fish in December.
Growth rates were highest in June and July, and again, declined thereafter. No surprise here — steelhead were moving downstream to smolt and became trapped, and then abundance and growth declined as the summer habitat conditions (intensified by the drought) became progressively more difficult.
During the study period only a small fraction of the tagged steelhead moved back into the upper part of the watershed. However, while most remained in the lagoon, a more substantial portion of the fish undertook daily movements out of the warm lagoon and into the lower part of the creek, where water temps were about 4-5 degrees (Fahrenheit) cooler. These movements were, as might be expected, most common during the hottest part of summer, and imply that fish were balancing their need to feed in the lagoon with their physiological requirement to reduce exposure to high temperatures and low dissolved oxygen levels.
As winter approached, juvenile steelhead were physiologically preparing to become saltwater ready and some fish apparently emigrated to the ocean once the sandbar was breached. However, the researchers found that a substantial portion of the fish did not leave for the ocean at that time. For example, 46 percent of the tagged fish did not enter the ocean and were detected elsewhere in the creek at later dates. Accordingly, while those fish missed their spring emigration window due to the sandbar, they likely were able to persist for another year in freshwater and smolt again the next year.
Over the past decade or so steelhead scientists have devoted more attention to analysis of lagoon habitats and their importance for steelhead. The results to date underscore the high value of such habitats for steelhead, especially in smaller, flashier stream systems, and suggest that steelhead can persist in lagoons even during times of extreme drought. In fact, some fish may grow at very rapid rates in estuarine habitats.
However, the studies to date also suggest that steelhead are able to survive in lagoons perhaps largely due to their ability to migrate to and from the creek. In the Scott Creek study, a substantial number of individuals made the best of a challenging situation by undertaking short daily migrations from the warm lagoon into the cooler water of the lower creek. In human terms, it’s not much different from moving from the sundeck to a cool air-conditioned room during a hot day. In lagoons which become disconnected from the stream, steelhead likely face much more dire prospects.
This study highlights some of the unique challenges that steelhead face in California, in particular the formation of sandbars at river mouths and the subsequent trapping of juveniles in the lagoons. As we saw in our previous Science Friday review, juvenile steelhead in California deal with some of the toughest temperature and oxygen conditions found anywhere in their native range. It is not an easy place for coldwater-dependent salmonids to make a living.
In summary, steelhead have evolved to exhibit a wide range of behaviors that allow them to live in places many other species could not. Estuarine habitats, especially terminal lagoons, are very important for steelhead in many California coastal streams. Lagoon formation and fish stranding may become increasingly common in coastal streams, even larger drainages, as the effects of our steadily warming climate are manifested. Ultimately, however, what the Osterback study really underscores is the value of connectivity between lagoons and their source creeks — one reason why Wild Steelheaders United and Trout Unlimited continue to invest heavily in cooperative, partnership-based projects that restore and reconnect habitats within priority steelhead watersheds.