Optimal Cage Depth for Bay d'Espoir Salmonid Aquaculture

N-01-06-003

Description

An important question for the Newfoundland Salmonid Growers Association (NSGA) is that of necessary net depth for optimal salmonid aquaculture performance efficiency during winter periods when sub-zero water temperatures impose undesirable ongrowing conditions. On the basis of their observations of the past few-years, the NSGA has hypothesized that, given sufficient aquaculture net depth, healthy salmonids will choose a position in the water column that avoids physiologically-difficult conditions. The present document describes the results of a research project undertaken to test this hypothesis and to determine the optimal net depth for over-wintering of Bay d'Espoir aquaculture-industry salmonids.

This research project was designed to examine Atlantic salmon (Salmo salar) and steelhead (Oncorhynchus mykiss) inventory survival in 10 m vs. 25 m deep nets during winter aquaculture operations. The species factor in this experiment was further divided to include year class (i.e., a typical ongrowing cycle in Bay d'Espoir takes place over an 20-month interval, meaning that at any given time there are two year classes of salmonids in the marine phase of the production cycle). For this experiment, 1st-year fish were from the 2000 year class; 2nd-year fish were from the 1999 year class. Odds-ratio tests confirm a significant difference in survival potential between the 10 m and 25 m nets for both salmon and steelhead. However, survival percentages show that this difference is attributable entirely to the 2nd-year fish for both species. Odds-ratios indicate salmon in a 10 m deep net are 1.65 times as likely to die during the winter as salmon in a 25 m deep net. For steelhead, the odds-ratio is 1.51. Consequences of overwintering are much more pronounced among the oldest/largest fish.

Highest mortality was observed among 2nd-year steelhead. Percent mortality among these steelhead in the 25 m net to mid-February was 29%. Mortality in the corresponding 10 m net was 45%. Of 20 moribund steelhead subjected to veterinary examination in mid-February, 18 were mature. Many of the moribund fish expelled eggs when handled. This suggests either that the 2nd-year steelhead were maturing or that those fish that were maturing were not able to survive the maturation process. Veterinary work confirmed that mortality among the moribund 2nd-year steelhead was preceded by osmoregulatory distress. Both cortisol and glucose levels showed the same pattern as blood osmolality.

For the interval of the experiment, Atlantic salmon spent most of their time (i.e., 86% of depth records) at ≤4.0 m. in the water column. Steelhead showed somewhat less affinity for these shallower depths but still tended to remain above the 10 m-depth limit of the industry standard nets. Recommendations to the Bay d'Espoir salmonid aquaculture industry as a result of this research are:

Use of nets of significantly >10 m for over-wintering of salmonids in high current areas in Bay d'Espoir is not economically justifiable for 1st-year salmonids. Net depths of up to 15 m may be useful for improving the economic performance of 2nd-year salmonids but only if maturation is controlled and sources of physiological stress can be identified and remedied.
The Bay d'Espoir salmonid aquaculture industry should investigate water-column use by salmonids during warm summer months to determine if nets of 15 m depth have any advantage over 10 m nets during the normal growing season.

Program Name

Aquaculture Collaborative Research and Development Program (ACRDP)

Year(s)

2001 - 2003

Ecoregion(s)

Atlantic: Newfoundland, Labrador Shelves

Principal Investigator(s)

Atef Mansour
Email: Mansoura@dfo-mpo.gc.ca

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