Unagi, or freshwater eel (Anguilla japonica and other Anguilla species), is extremely popular in sushi restaurants and Asian cuisine and has an annual market value of well over $1 billion. However, wild populations of the eels have declined dramatically due to wild harvest, climate change and other anthropogenic stressors. Now, European, Japanese and American eels are listed as endangered. Captive breeding has not been particularly successful, and eel-farming practices place significant pressure on other wild fish populations that are harvested for feed.
In this project, researchers tested the potential for the cultivation of an alternative species as a sustainable source of unagi. Cebidichthys violaceous, known as monkeyface pricklebacks (or monkeyface “eels”) are eel-like fishes that live in rocky intertidal and shallow subtidal habitats from Oregon to northern Baja California. These fish are primarily herbivorous, can live in small spaces and grow quite large — all qualities that make them attractive as a potential aquaculture species. Furthermore, monkeyface prickleback flesh is delicate, richly flavored and can be prepared in many ways. The researchers collaborated with industry partners, TwoXSea and Monterey Bay Seaweeds, to develop techniques to farm monkeyface pricklebacks at the Moss Landing Marine Laboratories Center for Aquaculture.
The researchers found that morphological differences seem to distinguish male and female fish, which means aquaculturalists will not need to rely on expensive ultrasounds to establish proper sex ratios for their broodstock populations. Initial results suggest thated adult male monkeyface pricklebacks have a larger supraorbital crest, among other landmarks around the eyes. Sexual dimorphism results were further verified by dissection.
Investigations of reproduction, spawning and larval growth revealed some challenges to raising monkeyface pricklebacks. While two different methods – providing fish with an environment that encourages natural reproduction and injecting fish with hormones that will induce spawning — prompted female fish to lay eggs, in initial trials male fish did not fertilize these eggs. Based on a hypothesis that interactions between male fish were limiting reproduction success, new trials were conducted that included varying sex ratios of fish. In the second reproduction experiment, two successful fertilized broods were produced in treatments with a single male and one or more females. Embryo development was documented in detail for the first time, and early larval landmarks were attained in captivity, including yolk sac depletion, first feeding, and development up to three weeks post-hatching.
Trials to determine the fish’s receptivity to soy-based feed formulae — an economical alternative to algal feeds — found fish growing well on several diets, both soy-based and without soy. Preliminary results indicated that the fish grew faster when fed soy pellets with a small amount of nori for taste, as compared to three other diets with larger amounts of seaweed.
Luke Gardner