Conservation News
Size Does Matter
By Dougald Scott
Size Does Matter
By Dougald Scott
When it comes to juvenile steelhead, size does matter. In a Master's Degree thesis published this summer, UCSC graduate student Morgan Bond clearly demonstrated that juvenile steelhead entering the ocean at a larger size had an extremely large survival advantage. Even more important, he showed that spending the summer and fall months in the natal stream's lagoon provided the growth advantage that improved ocean survival.
Scott Creek, located a few miles above Davenport, served as the site for this research. It is a relatively small watershed with about 14 miles of habitat for migratory steelhead and coho salmon. It differs from larger river systems to the north in that it does not form an extensive estuary where fresh water mixes with seawater. Larger estuaries with continuous connection to the ocean have long been known to be highly productive nursery areas for numerous fish and invertebrate species, including steelhead.
The Scott Creek estuary is typical of smaller, south coast streams that form fresh water lagoons only during the low flow periods of summer and fall. As springtime flows decline, a point is reached where the flow is not sufficient to breach the sand deposition on the beach. A sandbar develops, creating a slow-moving body of deep water cut off from its ocean connection. These lagoons are substantially warmer than upstream tributaries, but rarely too warm for salmonids. The sandbar washes out with the first substantial winter storms, reestablishing the estuary's connection to the sea.
Mr. Bond employed four strategies to investigate the extent to which juvenile steelhead utilize the lagoon habitat in Scott Creek, and their contribution to the returning adult population:
1. Determined the size and abundance of juvenile steelhead migrating downstream in the spring (peak migration is in late April or May).
2. Determined the size and abundance of the juvenile steelhead population remaining in the lagoon.
3. Used scale samples from returning adults to determine if the fish had entered the ocean directly on downstream migration, or had been reared in the lagoon before entering salt water.
4. Used passive integrated transponder (PIT) tags to monitor juvenile-to-adult survival rates in both the lagoon-reared and non-lagoon groups. When the data were collected and analyzed, they revealed growth rates in the lagoon to be nearly 10 times the growth rates observed in the upper watershed. This phenomenal growth is attributed to an abundant food source, mainly of amphipods (scuds), and resulted in downstream migrants doubling their length in just a few months of lagoon residence. Bond found the average length of downstream migrants in the spring to be about 4 inches, while the average length of the fish remaining in the lagoon through summer and fall was about 8 inches.
It appears that only a small fraction of the total downstream juvenile population remains in the lagoon. In 2003, 48% of the downstream migrants stayed, while only 8% remained in 2004. It needs to be pointed out that the 2003 figure is thought to be considerably less than the observed 48% because of a problem with the capture net. Downstream migrant data for the years 2002 and 2005 were not used because high spring flows prevented effective sampling of the populations.
Scale and PIT [transponder] tag analysis on returning adult steelhead confirmed that juvenile steelhead entering the sea at a larger size had a definite survival advantage. The majority (56%) of returning adults were 8 inches or longer upon ocean entry as juveniles. The cutoff for ocean survival was about 5.5 inches. Keep in mind that the entire downstream migrant population averaged only 4 inches.
Where did the larger juveniles come from? As noted above, lagoon-reared steelhead entered the ocean at an average size of 8 inches. As a result of their size advantage, they comprised 85% of the returning adult population despite only representing somewhere between 8% and 48% of the entire juvenile population.
This study makes it clear that lagoon habitat is extremely important in producing juveniles of sufficient size to survive life in the ocean and return as adults. Lagoon waters in Scott Creek comprise less than 3% of the habitat available to steelhead, yet the vast majority of returning adults are products of that environment. Bond concludes: "This indicates that coastal estuaries may be more important to steelhead persistence in the southern portion of their range than previously thought, and [lagoon] degradation could have drastic implications for steelhead populations already listed as threatened or endangered. Indeed, restoration of coastal estuaries may be an effective method of returning steelhead to their historic population levels in these watersheds."
For good reason the emphasis of steelhead conservation effort has focused on spawning habitat and access to it; without successful spawning, there will be no steelhead. However, the current study instructs us to broaden our view and take into account the fact that most of the returning adult steelhead in local streams were probably reared in lagoons or estuaries. These habitats have suffered as much, if not more, environmental degradation than the upper reaches of their watersheds.
The present size of the Scott Creek lagoon is much smaller than it was before construction of Highway 1 across the lagoon in the late 1930's (verified by historic aerial photographs). Roads and bridges have had similar impact on other county streams. The San Lorenzo lagoon and estuary was historically much different from its current channel with levees built in the late 1950's. The Aptos Creek lagoon has been reduced in size and compressed between two concrete walls. Both of these lagoons also suffer from severe sedimentation and pollution, as does the Soquel Creek lagoon.
In spite of these problems, steelhead persist in our local urban lagoons. Just today, I could see many 8-10 inch steelhead below the San Lorenzo Park footbridge. Last month, a San Jose fly fisher alerted me to many 8-10 inch steelhead near the Spreckels Street bridge in Aptos Creek. Think of what effect improving these habitats would have on adult steelhead populations in each watershed. Over the next few months, I will be working with local, state and federal agencies responsible for local lagoons to find out if there is a lagoon restoration project our club could realistically undertake. I think we can make a difference and I will report back to you.
Addendum #1: In the past, some of our urban lagoons were managed by intentional breaching in order to improve beach access, or to control for high bacteria counts. This is now illegal. If you observe anyone attempting to breach a lagoon, immediately notify California Fish and Game (1-888-DFG-CALTIP). Fishing in any of our county lagoons is illegal from March 8 through December 1 (check 2007 regulations for any changes). Report poachers to the same number.
Addendum #2: Mr. Bond's thesis is an elegant piece of work, and well worth reading in toto. You can download it at: www.cemar.org/pdf/Bond%20Thesis%20Lagoon%20Rearing.pdf.