– – – – – – – –
BY KERSTIN WASSON

June 2003: Biologists have long extolled the virtues of estuaries as nursery habitats for marine fish. However, there is surprisingly little quantitative evidence characterizing the nursery role of estuaries. Certainly baby fish have been observed many times, apparently enjoying the shallow, warm, nutrient-rich waters of the slough and other estuaries. But to fit a rigorous definition of a nursery, the estuary actually has to contribute disproportionately to the adult population, in comparison with other coastal areas.

Researcher Jennifer Brown (top) and an assitant haul in an otter trawl, the open-mouthed net they used to capture English sole and speckled sanddab.

A few years ago, when our graduate research fellow Jennifer Brown began her research, I reported on her elegant approach to assessing the importance of estuaries as nurseries for flatfish. Now she has successfully completed her doctorate, and here’s what she found out.

To determine where ocean-dwelling adult fish spent their infancy has been challenging in the past. Tagging juveniles in coastal habitats up and down the coast and then trying to recapture the adults would be nearly impossible. Instead, Jenn used a neat trick – she let the habitats tag the fish for her. She determined that the ear stones of flatfish incorporate chemical signatures of their environment. Like trees, ear stones (called otoliths) add rings of growth each season. It was therefore possible to start with an adult’s ear stone and then cut away the outer layers, ending up with just the innermost part corresponding to the juvenile period. Jenn first groundtruthed the method, classifying ear stones from fish of known juvenile origins. She developed a statistical method that could correctly identify the nursery habitat (estuarine vs. coastal habitat) about 80% of the time.

Jenn then drew upon connections with local fishermen and researchers to collect hundreds of adult Speckled Sanddabs and English Sole from the Monterey Bay. She carried out the chemical analysis of their inner ear stones, and applied her statistical wizardry to determine where their nursery habitats had been. Since the Slough accounts for about 5% of available shallow coastal habitat in the Monterey Bay area, anything more than 5% of the adults having spent their juvenile period there would fit the definition of a nursery: disproportional contribution to the adult population.

To her amazement, she found that about 55% of the adult flatfish she captured had spent their juvenile period in Elkhorn Slough! This is an order of magnitude greater than would be predicted simply based on area. So she has provided the first rigorous evidence that our estuary does indeed serve as an important regional nursery for fish.

Why does the Slough contribute disproportionately to adult flatfish populations in the Monterey Bay? Jenn also carried out some experiments to address this question. She evaluated habitat quality by caging juveniles in Elkhorn Slough vs. along the open coast. After a month, she found that the flatfish in the estuary had grown substantially more than those on the coast, perhaps due to greater temperatures or richer food supplies. She additionally collected naturally occurring juveniles from multiple estuaries and other coastal areas, and found that the earstone rings were more widely spaced in estuarine habitats, implying higher growth rates.

This research shows that the Slough is an important nursery habitat, and now there are strong data to back up the pleasant image of baby fish delighting in the warmth and devouring the delicacies of our estuarine habitats.