REDUCED TANK-CLEANING FREQUENCY, BUT NOT CO-CULTURE WITH SEA CUCUMBERS, IMPROVES SURVIVAL OF LARVAL SABLEFISH Anoplopoma fimbria

The larval stage for marine fishes is labor intensive and characterized by high mortality. These challenges reduce profitability and limit numbers of fingerlings available for further rearing. Significant labor is dedicated to tank cleaning, to remove organic matter which enters rearing tanks in the form of feeds, feces, dead larvae, and turbidity agents such as algae. However, tank cleaning protocols are often a product of tradition or intuition, rather than replicated experiments. In this study, we compared three tank cleaning protocols for larval sablefish.

Cleaning activities were typical for marine larviculture, including siphoning of tank bottoms, scrubbing of tank walls, and spraying of center standpipe screens (see figure below). The control treatment approximated our traditional sablefish protocol, while a "reduced frequency" treatment conducted most of the same cleaning activities, but at a lower frequency. A third "multi-trophic" treatment cleaned at that same reduced frequency, but also co-cultured sea cucumbers (Apostichopus japonicas) that we hypothesized would help remove excess organic matter and improve larval survival.

The reduced frequency treatment showed improved larval sablefish survival relative to control, but sea cucumbers drastically reduced larval survival in the multi-trophic treatment. Water quality measures did not differ among treatments, and microbial community was affected by date but not treatment, supporting the conclusion that cleaning frequency can be reduced. This reduced cleaning protocol allows for less labor without negative impacts on larval survival.