Research Summary

Determining Contribution of Juvenile Habitats:

Some fish species spend the first months of their lives in juvenile habitats, such as estuaries and the shallow sandy areas of the open coast. At the end of this period, juveniles move into deeper water and join adult populations offshore. Scientists and managers have wanted to evaluate the relative contribution of estuarine and coastal juvenile habitats to adult populations. However, they have been hampered by the difficultly of tracking the movement of juveniles between these two habitats. Recently, Jennifer Brown, a graduate research fellow at the ESNERR has begun to apply a new technique to answering this important question.

Jennifer is using the chemical composition of fish ear stones (otoliths) to track the movement of fish between juvenile and adult habitats. Otoliths can be used as tracking devices because: 1) chemicals in the water column are incorporated into the ear stones; and 2) the chemistry of estuaries differs from that of coastal habitats. Jennifer has spent three summers (1998-2000) collecting juvenile English sole and speckled sanddabs from estuaries and coastal habitats throughout central California, including Elkhorn Slough and the Monterey Bay. She has analyzed the chemical composition of the otoliths from many of these samples and found that distinctive chemical signatures exist in the otoliths that correspond to the juvenile habitat in which a fish was collected.

The next step in this exciting detective story is the collection and analysis of otoliths from the adult populations located offshore. Jennifer is currently excising the juvenile part of the adult's otolith, and carry out the necessary chemical analysis. Afterwards, she will finally be able to answer the question that has so long puzzled scientists. She will know what proportion of these adult fish grew up in the coastal waters vs. the estuaries. This knowledge will help us to better protect fish populations by monitoring the health and persistence of appropriate juvenile habitats.

Comparing Growth Rates in Juvenile Habitats:

If you are a juvenile flatfish, is it better to grow-up in an estuary, such as Elkhorn Slough, or on the sandy coast of Monterey Bay? Many researchers believe that estuaries are the preferred habitat because they have more food and warmer waters - a combination that should allow fish to grow bigger faster. Jennifer is trying to determining if this belief holds true for Elkhorn Slough. However, answering this question can be tricky because it is difficult to determine growth rates of wild fish. So Jennifer is using two methods to measure the growth rate of juvenile flatfish in Elkhorn Slough and Monterey Bay.

In the second method, juvenile flatfish are collected in the estuary and on the coast. The otoliths from these fish are examined and the width of daily rings in the otoliths are measured. The width of these rings reflect the growth rate of the fish. The advantage of this method is that it allows the comparison of growth rates of wild fish that were foraging under natural conditions. The disadvantage is that increment widths in the otoliths are only a proxy for measuring the overall growth of the fish's body.

Jennifer completed the caging experiment in the summer of 2000 and the results indicate that juvenile flatfish grew faster in the estuary. She is now in the process of measuring daily increment widths in the otoliths of juveniles collected in Elkhorn Slough and Monterey Bay. We at the ESNERR, anxiously await the results.

Determining growth rates in juvenile habitats is key to understanding the relative importance of these two habitats in producing large and healthy juvenile fish. Though neither method is perfect for determining growth rates of wild fish, by comparing both methods, we should get closer to understanding growth conditions in juvenile habitats.