High connectivity of salmon farms revealed by aggregation, residence and repeated movements of wild fish among farms

Abstract

Marine fish farms are widespread in coastal waters throughout the world, yet how they modify the movement patterns of wild fish species is largely unknown. We determined the spatiotemporal distribution of saithe Pollachius virens in a fjord system with intensive salmon cage aquaculture in Norway. Abundances of 8000 to 18 000 saithe were estimated around 2 salmon farms in the fjord using an underwater video system. Residence of saithe around fish farms and movements among farms and throughout the fjord were studied using implanted acoustic transmitters and an extensive array of automatic receivers. Of the saithe equipped with acoustic tags, 63% were observed daily at one or more of the 3 farms in the fjord over a 3 mo period. When resident at a farm, saithe spent 8 to 10 h d -1 close to the sea-cages. Periods of residence at specific farms were interspersed with rapid and frequent movements to adjacent farms 1.6 to 4.7 km away. Of 24 tagged saithe, 15 moved among farms 2 to 21 times during the 3 mo period. If the movement patterns of the tagged fish are representative of the movements of untagged saithe, we estimate that fish from 2 different farms resulted in a total (± SE) of 167 112 ± 41 764 and 7768 ± 1831 inter-farm movements during the 3 mo period. Thus, fish farms should be considered as connected, not only through ocean currents, but also through wild fish movements. If saithe share pathogens with farmed salmonids, their behaviours imply that they have the potential to act as vectors for diseases and parasites among salmon farms.KEY WORDS: Aquaculture · Sea-cage · Aggregation · Movement · Pollachius virens · Tagging · Tracking Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 384: [251][252][253][254][255][256][257][258][259][260] 2009 sea lice infestations of wild salmonids (Bjørn et al. 2001, Krkosek et al. 2007). An additional impact involves the attraction and aggregation of large amounts of wild fish by salmon farms (e.g. Carss 1990, Dempster et al. 2009). The importance of this phenomenon for the management of wild fish populations and fish farming environments is gathering momentum (Dempster & Sanchez-Jerez 2008).Wild fish are resident near farms over a sufficient period to drive some physiological changes, such as modifying diets, fat content and tissue fatty-acid distributions (Skog et al. 2003, Fernandez-Jover et al. 2007). This may also increase levels of pollutants such as mercury in the tissues of long-term residents beneath farms (deBruyn et al. 2006). Biological evidence (increased liver somatic index) in farm-associated saithe Pollachius virens (Skog et al. 2003) and the results of tagging studies in southern Norway (Bjordal & Skar 1992, Bjordal & Johnstone 1993 suggest that saithe reside in the vicinity of specific farms for several months. Because of this, wild fish may also act as a source of pathogens to cultured fish in farms (e.g. Sepúlveda et al. 2004). However, little direct evidence exists to assess whether wild fish trans...

Host density thresholds and disease control for fisheries and aquaculture

“…These characteristics of the marine environment likely underlie the observation that diseases spread at rates an order of magnitude faster in the oceans than they do on land (McCallum et al 2003). These characteristics, in addition to connectivity of farms by wild fish movement (Uglem et al 2009), suggest that the spatial scale for host density thresholds in the oceans is likely larger than an individual farm and likely occur at regional scales such as large embayments, fjords, or archipelagos.…”

“…Infection pressure, however, likely declines rapidly with distance from source population. In addition, microparasites may spread among farms over broad spatial scales due to the movement of wild fish populations (Uglem et al 2009). For example, herpes epidemics of Australian pilchard stocks in 1995 and 1998-1999 spread at rates of 30 km d -1 or 10 000 km yr -1 (Jones et al 1997, Gaughan et al 2000, McCallum et al 2003.…”

“…Vaccines for parasitic disease, particularly sea lice, are an important research area but have not yet been successfully developed and implemented (Raynard et al 2002, Boxaspen 2006, Alvarez-Pellitero 2008 SPATIAL SCALE OF THRESHOLDS AT SEA Domesticated marine populations exist at nested spatial scales, ranging from a net pen, raft, cage, or rope, to a farm or a collection of farms in an embayment or region. Furthermore, there may be mobile wild host populations in the surrounding environment that vary in abundance through time and space, and which can connect distant aquaculture facilities (Uglem et al 2009). The spatial scale which defines the relevant host population upon which host density thresholds might act is likely determined by the disease connectivity within and among these host subpopulation units.…”

Coastal salmon farms attract large and persistent aggregations of wild fish: an ecosystem effect

“…The estimates assume that fish outside this limited area are not farm-associated, which is unlikely to be the case for mobile pelagic species such as Pollachius virens and S. scombrus. Ultrasonic tagging and tracking of farm-associated P. virens in Øksfjord revealed that P. virens may be resident at farms for weeks to months or may move among different farms and natural locations several kilometers apart (Uglem et al 2009), thus using farms as frequently visited locations within a wider home range. If P. virens regularly move in and out of the attractive area of farms, this process would mask the true numbers of P. virens that regularly interact with farms, which are likely to be higher than our estimates restricted to the area of the cages.…”

“…Aggregation of wild fish adjacent to caged fish may also increase the risk of transfer of shared pathogens between the 2 groups (Diamant et al 2007, Uglem et al 2008 or, possibly, to nearby fish farms if wild fish migrate between farms (Uglem et al 2009). The concentration of wild fish at fish farms may assist in nutrient dispersal at farming sites (Vita et al 2004) and could also increase the vulnerability of fish to capture, which has led to the proposed implementation of nofishing zones around sea-cage farms .…”