Monterey Bay Salmon & Trout Project

The Importance of Lagoon Habitat to Steelhead Survival on the Central Coast
By Morgan Bond, NOAA-NMFS

As many of you may know, I have been working with Sean Hayes on the NMFS Scott Creek Steelhead and Coho monitoring project for several years now. Shortly after being hired to assist Sean with field and lab work, I began to see some interesting trends in our data and also from the work of Prof. Jerry Smith and others, that pointed to the small Scott Creek lagoon as being very valuable habitat for juvenile Steelhead rearing.

In fact these trends were interesting enough to encourage me to enter the graduate program at UC Santa Cruz and explore the importance of the lagoon habitat to Steelhead survival as my Master’s thesis. As any MBSTP volunteer who has helped with the capture of Scott Creek Steelhead can attest, we take scale samples from any fish we get our hands on. Scales are particularly useful, because like the growth rings on a tree, scales record a history of fish growth. Using scales we can determine fish age (both ocean and juvenile freshwater age), spawning history, and now, using digital photography and computer aided precision measurement, we can determine how fast a fish was growing at each year of its life.

This is particularly important because we have discovered that growth of juvenile Steelhead in the lagoon is, at times, up to ten times faster than that of fish in the upper watershed (and some times even faster than fish in the hatchery). We also know that during the spring smolt run when young fish are moving from the upper watershed to the ocean, between 10 and 50 percent of these emigrants “choose” to remain in the lagoon and not go to sea. Fish that go to sea are fairly small, at about 100 mm (4.5”) in length. However, fish that remain in the lagoon throughout the summer have the opportunity to experience the high growth rates the lagoon affords, and will remain at least until the first winter storms open the lagoon.

In the intervening months, lagoon fish roughly double in size compared to their spring emigrating counterparts. I have been able to look at thousands of scales over the last several years from juveniles of both upstream and lagoon habitats. In a blind test I can assign a juvenile to its habitat of origin (either upstream or lagoon) with about 86% accuracy, based purely on the growth rate driven spacing of characteristic marks on the scales. Luckily, marks on the scales that were created when young Steelhead were growing are permanently recorded and can be read from adult scales.

In Scott Creek we operate a small weir that allows the capture and sample of about 75% of the returning adult population. I took scales from these adult Steelhead and analyzed them using identical procedures to the juvenile scales. Two analyses showed some striking results. First, I was able to back- calculate the size at ocean entry for each returning adult. This allowed me to compare the size at ocean entry of the survivors (returning adults) to the spring emigrating juveniles and

“…we have discovered that growth of juvenile Steelhead in the lagoon is, at times, up to ten times faster than that of fish in the upper watershed…"

fall lagoon residents for each year class. What I found was that few fish entering the ocean at less than 150 mm (6”) returned as adults. However, most of the spring emigrants are smaller than this survival threshold, and most lagoon residents are much larger than this minimum size before ocean entry in the fall.

Secondly, by looking only at the juvenile portion of each adult scale, I could use the same analysis of characteristic growth marks, or circuli, that we used to classify juveniles, to assign each returning adult as either upstream reared, or lagoon reared. As it turns out, the analysis indicates that 80% of the returning adults used the lagoon habitat prior to ocean entry. Remember though, that only between 10% and 50% of the juveniles from a given year even use the lagoon habitat, which further accentuates the importance of the lagoon as a critical habitat necessary for many of the juveniles to achieve a size large enough to compete and survive in the ocean.

Finally, we have been tagging both groups of fish (spring migrants and fall lagoon residents) with permanent, implantable, Passive Integrated Transponder (PIT) tags. These tags last essentially forever, and allow us to measure ocean survival of both types of fish. To date, 17 adults have returned carrying tags. All 17 were originally tagged in the lagoon during the summer and fall of 2003 and 2004. Tag returns have helped to verify what we have observed in scale growth patterns and further illustrate the importance of the lagoon habitat. Although we are currently only studying the Scott Creek watershed, the important role the lagoon is playing in this watershed highlights the need for studying other lagoons on the central coast, many of which are impacted by alterations to the habitat. Restoration or protection of lagoons may be critical to Steelhead survival in our area.

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