COS 37-2 - Tracing exploitation of a fish egg boon in marine food webs

Abstract: Coordinated spawning of many marine animals releases millions of planktonic eggs into the environment creating pulsed resources for egg consumers, called egg boons. Eggs are especially rich in essential fatty acids and may be an important lipid subsidy to egg consumers. While a variety of marine animals consume eggs, the role of egg boons in energy transfer through food webs has received little attention. Our research exploited an annual egg boon produced by a spawning aggregation of red drum (Sciaenops ocellatus) near Port Aransas, Texas.

In a combination of field sampling and laboratory experiments, we measured fatty acids and bulk nitrogen stable isotope (δ¹⁵N) to define trophic links between the egg boon and a selection of lower-trophic-level taxa. Firstly, we conducted controlled feeding experiments to validate the application of fatty acid and δ¹⁵N tracers of red drum egg consumption to potential egg consumers. We conducted feeding experiments with a several invertebrate and fish species, and collected these animals from the field during and outside the extent of the egg boon. We then utilized the identified biomarkers of red drum egg consumption to examine the hypothesis that egg boons provide a pathway through which essential fatty acids are redistributed counter to the traditional direction of trophic flow.

Results from feeding experiments indicated that consumers took on fatty acid characteristics of red drum eggs. Fatty acid biomarkers of egg consumption were species-specific. Further, consumption of eggs with high δ¹⁵N led to δ¹⁵N enrichment in some egg consumers. For the field-collected samples, we expect fatty acid biomarkers of egg consumption to be significantly different in egg consumers collected during the egg boon when compared to those collected from outside the spatial or temporal extent of the egg boon. We also expect enrichment of δ¹⁵N in field-collected egg consumers from the egg boon but not in those collected outside of the boon. This research examined how fish egg consumption redistributes essential fatty acids within food webs, and provided a context for considering potential controls and trophic bottlenecks that cannot be explained from the traditional element-limitation perspective.