Technological solutions and operational measures to prevent escapes of Atlantic cod (Gadus morhua) from sea cages

Abstract

Escapes of cod (Gadus morhua) from sea cages represent an economic problem for farmers and a potential environmental problem. We estimate that 0^6% of cod held in sea-cage farms in Norway were reported to have escaped each year from 2000 to 2005, which is a high proportion compared with salmon.We interviewed employees at 19 coastal sea-cage cod farms in Norway to investigate both how and why cultured cod escape and to document cage handling and management strategies that were e¡ective in minimizing escapes. Based on the interviews, we describe ¢ve working hypotheses that may explain why a greater proportion of cod than salmon escape: (1) cod are more willing to escape than salmon; (2) cod bite the net cage and create wear and tear; (3) net cages have insu⁄cient technical standards for cod culture; (4) cod are placed in sea cages at considerably smaller sizes than salmon; and (5) cod are more popular feed for predators. Preliminary testing of the hypothesis that cod bite netting and create holes was done by placing pre-damaged net panels with cut twines and control panels inside sea cages. Holes in the pre-damaged net panels increased in size over a period of 3 months. The type of damage indicated that biting of netting twines was the likely cause. Escape of cod from sea-cage ¢sh farms H Moe et al. Aquaculture Research, 2007, 38, 91^99 Aquaculture Research, 2007, 38, 91^99 Escape of cod from sea-cage ¢sh farms H Moe et al.

Net-biting and escape behaviour in farmed Atlantic cod Gadus morhua: effects of feed stimulants and net traits

“…The present study demonstrates how escaperelated behaviours are affected both by net traits and feed stimulation, and the results are consistent with previous findings of net-biting (Moe et al 2007, Hansen et al 2008, confirming that Atlantic cod frequently inspect and bite the net, and try to escape through small holes in the net wall. Net failure has accounted for approximately two-thirds of the reported escape incidents in Norway, and biting from predators or fish, abrasions, collisions and handling procedures are among the most common reasons for holes in the net (Jensen et al 2010).…”

“…This result indicates that irregularities in the net wall alone may stimulate the fish to bite. Net-biting may be regarded as a part of the common behavioural repertoire of farmed cod, as suggested by Moe et al (2007) and Hansen et al (2008). Even though the individual biting rates were low, a corresponding biting rate in a commercial sea cage would result in a high amount of biting due to the large quantity of fish.…”

“…Farmed cod are most likely more prone to escape from net cage farms than for instance Atlantic salmon (Moe et al 2007, Hansen et al 2008. Jensen et al (2010) studied cases of reported escapes brought forward to the Norwegian Directorate of Fisheries for the years 2006 to 2009, and among the causes for the different escape events, biological and unknown causes accounted for 25 and 27% of the total number of escaped cod, respectively, whereas in salmon farming, these 2 causes account for less than 5% of the lost fish.…”

“…Jensen et al (2010) studied cases of reported escapes brought forward to the Norwegian Directorate of Fisheries for the years 2006 to 2009, and among the causes for the different escape events, biological and unknown causes accounted for 25 and 27% of the total number of escaped cod, respectively, whereas in salmon farming, these 2 causes account for less than 5% of the lost fish. Escape of farmed cod was not recorded systematically until 2004, but Moe et al (2007) A cod escape event may often be a result of major technological and operational failures, but escape through small openings in the net wall has also frequently been reported (Jensen et al 2010). Small openings in the net pen may be a result of mechanical damage, but it is also reported that farmed cod bite or nibble the netting and thus add to the wear and contribute to the creation of holes (Moe et al 2007, Hansen et al 2008.…”

“…Escape of farmed cod was not recorded systematically until 2004, but Moe et al (2007) A cod escape event may often be a result of major technological and operational failures, but escape through small openings in the net wall has also frequently been reported (Jensen et al 2010). Small openings in the net pen may be a result of mechanical damage, but it is also reported that farmed cod bite or nibble the netting and thus add to the wear and contribute to the creation of holes (Moe et al 2007, Hansen et al 2008. The high number of fish in a net pen implies that the total impact of net-biting may represent a problem (Hansen et al 2008).…”