The ultimate goal of artificial upwelling is to grow fish for human consumption in a sustainable way by boosting ocean productivity through deep water nutrients. But what kind of deep water creates the planktonic community most favorable for fish?
An inconspicuous small pelagic fish from the local ecosystem proved ideal to address this question. This so-called silverside feeds in the water column on small invertebrates, in particular shrimp like copepods. These are also the favored food of baby fish in general and of global fisheries species such as sardines and anchovies. In other words, given its function within the food web, our study species may reveal overarching principles that also apply to fishes of high commercial importance.
Two weeks into the experiment, it was time to complete our food web. Wild fish were caught nearby using nets and light traps. At sunset, after a long day of sorting, we introduced two distinct size classes as new members of our mesocosm community: baby fish (0.8 cm) and young teenagers (1.7 cm). This strategy may allow us to answer two key questions at once. On the one hand, our baby fish feed on the tiniest invertebrates, just like the babies of most other fish species. A planktonic community (created by a certain upwelling treatment) that helps baby fish to survive may improve the replenishment of fish stocks. On the other hand, our teenage fish can catch larger invertebrates similar in size to what an adult sardine prefers. Their performance in the mesocosms may thus link to the grow-out of fish to adulthood or, in fisheries terms, the build-up of protein for our dinner plates.