Error message

Deprecated function: The each() function is deprecated. This message will be suppressed on further calls in menu_set_active_trail() (line 2385 of /srv/data/web/vhosts/www.indiseas.org/htdocs/includes/menu.inc).

West Coast of Scotland - Ecology

The general seasonal trends in chlorophyll standing stock for both the sea-lochs and coastal areas are known but analyses of longer-term trends suffer from a lack of data. However, the CPR phytoplankton colour index (PCI) shows a shift in patterns of chlorophyll standing stock around the mid-1980s with the largest changes in the shelf waters. This change has previously been noted for adjacent sea areas such as the North Sea. Furthermore this change coincides with the recent warming of the surface waters which in turn appears to be linked with the amount of Atlantic water entering the area.

The abundance of copepods in ICES Division VIa, as monitored by the CPR, has declined over the times-series (1948-2008). In the offshore waters changes in total abundance are mainly driven by shifts in the numbers of Acartia spp. whilst variability in Para-Pseudocalanus spp. tends to dominate in the shelf waters. Food availability for the early life stages of fish is generally thought to be an important factor affecting their year-class strength. The decline in Calanus finmarchicus and euphausiids in the North Sea is frequently cited as a reason for reduced recruitment of cod in that area. Although there have been changes in the abundance of Calanus finmarchicus in the shelf waters of ICES Division VIa, much larger declines in the abundance of Para-Pseudocalanus spp. have occurred. This may actually be of more importance for the early gadoid larvae to the west of Scotland as Para-Pseudocalanus copepods are known to be important prey for larval gadoids in other areas. However this evidence is circumstantial and based on limited CPR sample routes. Improved monitoring of the zooplankton in the gadoid spawning areas as well as research into the diets, feeding success, growth and condition of larval and juvenile gadoids in waters to the west of Scotland is needed.

In Scotland, juvenile Atlantic cod and whiting occupy the nearshore environment for most of their first year of life, and move into deeper water as age-1 fish during their first winter and second summer. Haddock are present in the nearshore only sporadically, and exhibit no consistent use of the nearshore at any point during ontogeny. Juvenile whiting and cod in the nearshore environment associate with complex habitats, including eelgrass, rocky substrate, macroalgae, and reef habitat. Loss of these habitats can negatively affect juvenile abundance of these species.

The nearshore environment does not appear to exert any effect on adult haddock abundance or distribution. Natural conditions or human impacts in the nearshore environment can have large effects on juvenile haddock abundance. The resulting effects of year-classes strength can be traced through adult populations for up to six years following their settlement year.

Threats to juvenile cod and whiting abundance, such as habitat loss, vulnerability to capture as juveniles by Nephrops (Norway lobster) fishing trawls, pollution, salinity changes, and predator abundance can be monitored and, in some cases, mitigated through management schemes. As the loss of suitable nearshore habitat appears to have negative effects on cod and whiting juvenile abundance, the potential benefits of habitat restoration or artificial reef implementation should be explored further.

The stocks of the WC of Scotland are assessed by ICES and managed by Marine Scotland. However, the assessment methodology is in question with very little agreement between managers and fishers. In general the surveys support the stock trends estimated from the ICES assessments for cod and haddock, with some disagreement between the two analyses for whiting. The survey based assessment (SURBA) results for cod in Division VIa supports the perceived long term decline in the stock with some stabilization in recent years. Trends in recruitment, SSB and Z show the most similar patterns between the working group assessment and SURBA outputs for haddock; this may be attributed to the fact that commercial catch data is excluded from the assessment for haddock from 1994 onwards.