RECONSTRUCTING FISH ECOLOGY FROM OTOLITH GEOCHEMISTRY: PAST AND PRESENT

Abstract

Effective fisheries management, with the goal of sustainable fishing, requires that scientists understand as much as possible about a population’s ecology. Ecological data regarding feeding habits are especially difficult to quantify due to methodological constraints. Current dietary/trophic reconstruction methods such as stomach content analysis and isotopic measurements of tissues including liver and muscle d15N, which indicate relative trophic position in an ecosystem, only resolve diet on timescales up to three months. In contrast, fish ear stones, known as otoliths, accrete over the entire life of the fish and are metabolically inert once precipitated. This study develops a new method that measures the d15N of the organic material in early and later life history otolith layers in order to observe trophic changes through a fish’s lifespan. Aquaculture cod fed a diet of constant d15N and Atlantic croaker otoliths that had gone through a controlled diet d15N switch were used to validate this method. Otolith powder for early and later life history was obtained using an automated MicroMill to drill out specific sections of otolith into powder. Isotopic measurements of the carbonate bound organic material were made with the previously-established persulfate denitrifier method. The micromilling method results in reproducible measurements that accurately tracked a dietary switch. Unfortunately drilling out early and later life history otolith material and measuring the d15N of the separate samples is an accurate but imprecise method that requires ample time, an abundance of samples and otoliths with a large cross sectional surface area in order to drill enough material and be sure the material is being drilled from different temporal points. For improvement of the method, future work will need to determine how long it takes for a fish’s metabolism to adjust to a dietary shift and how long that adjustment takes in terms of amount of otolith aragonite precipitated. Furthermore, this research would be aided by studying the amino acid abundances in otolith bound organics.