Extent and ecological importance of escape through spawning in sea-cages for Atlantic cod

Abstract

The culture of certain fish species to sizes at which they can reproduce has led to the escape of fertilised eggs or 'escape through spawning'. To investigate the extent and ecological importance of spawning in sea-cages for Atlantic cod Gadus morhua (L.), we (1) evaluated the extent, frequency and timing of spawning in cod culture; (2) analysed the quality of eggs released from farms in terms of variation in fatty acids; (3) modelled the distribution of eggs and larvae from a commercial cod culture site; and (4) predicted the post-escape survival of eggs through summarizing existing knowledge on survival rates of different life stages. Collectively, our results indicate that cod farming has the potential to produce large amounts of eggs and larvae through spawning in cages, with numbers of eggs spawned being 4 to 5 times higher in the second than in the first year. Our scenarios suggest that a typical sea-cage with 60 000 fish may produce 1.4 to 21 tons of 3 yr old first generation farmed cod through spawning in sea-cages. The quality of escaped eggs and larvae is likely to be sufficient for larvae to survive until the first feeding, while survival until adulthood, though difficult to predict, may be high under favourable conditions. Simulations indicate that eggs and larvae from farms may mix with those of wild fish during the spawning season, and thus experience comparable larval environments. However, several implementable management measures exist that will diminish the extent of egg escape in future cod farming.

Farmed cod escapees and net-pen spawning left no clear genetic footprint in the local wild cod population

“…Frequent pen wreckage and escape incidents raised the similar concerns to those with sal monid aquaculture regarding harmful effects of genetic introgression by escapees into locally adapted wild populations (Bekkevold et al 2006, Moe et al 2007. These concerns are supported by the findings that adult escaped cod have appeared on the spawning grounds of wild relatives (Wroblewski et al 1996, Uglem et allandlocked fjord Hei marks pollen near Bergen, Norway, cod larvae from net-pen spawning were found up to 8 km from the net pen (Jørstad et al 2008), thus representing a potential genetic introgression of farmed cod into natural cod populations (Bekkevold et al 2006, Jørstad et al 2008, Glover 2010, Uglem et al 2012. Jørstad et al (2013) showed that genetically marked cod dispersed through out a fjord system, and documented the presence of juveniles and successful reproduction either by spawning in net pens or by escapees participating on local spawning grounds.…”

“…To the degree that such transport takes place, it would tend to reduce, but not exclude the pos sibility of a genetic introgression in the local cod stock by net-spawning. Uglem et al (2012) simulated egg dispersal for a 46 d period after spawning for this cod farm and fjord system, and indicated that the prob ability of eggs being carried out of the fjord by the estuarine circulation was 60 times higher for eggs spawned from the fish farm compared to those spawned from the main wild cod spawning site located further inwards in the fjord. During the entire pelagic period (eggs, larvae and pelagic codlings) until settling, and which in this study also includes the time of the annual spring flood, the probability of drifting out of the fjord is likely to be higher.…”

Prevalence and survival of escaped European seabass Dicentrarchus labrax in Cyprus identified using genetic markers

“…However, studies in Cyprus indicate that substantial numbers of seabass females do mature in the seacages under normal farming conditions (Brown et al 2014) and this represents a potential source of escape (ICES 2006). Egg production from such spawning could be very significant but survival is likely to be extremely low, particularly beyond the larval stage, and the true impact on natural populations is hard to determine (Uglem et al 2012).…”

Towards cod without spawning: artificial continuous light in submerged sea-cages maintains growth and delays sexual maturation for farmed Atlantic cod Gadus morhua

“…The spawning period of wild cod has evolved to concentrate in the window from February to May (Kjesbu & Kryvi 1989, Berg & Albert 2003, which coincides with the growth period of the nauplii of Calanus finmarchicus, one of the major prey items for cod larvae. Thus, temporal mismatch between cod larvae and prey and poor larval survival are far more likely for farmed cod spawned in summer (Uglem et al 2012). In addition, cod larvae have an upper thermal tolerance of around 12°C (Geffen et al 2006), which was exceeded in the submerged cages from early July until September and may be ex ceeded in coastal waters in summer more broadly where cod farming occurs.…”

“…Releases of fertilized eggs from spawning farmed cod may also negatively impact local cod stocks (Jør -stad et al 2008, Uglem et al 2012. Offspring can intersperse with wild populations as juveniles, recruit to spawning populations and successfully spawn in the wild (van der Meeren et al 2012).…”