Hi Guys, apologies for the delay in responding, busy at sea an all that.
Well where to start, firstly I think some of the responses to the query indicate just how much anglers have become educated in marine science. This is because we have a real vested interest and its so encouraging so see it used in a sensible approach.
The big problem nowadays is that unless a species is of significant commercial or conservation interest it is unlikely to warrant much research, despite its ecological importance, so I guess there is no hard and fast answer to the query, all we can do is summise.
I\'m not totally convinced that sea temperature increase as a result of global warming is the answer to all the changes that are occuring in our northern waters, it has been suggested that due to sea temperature rise the cod are not able to spawn as successfully as before, despite the fact that in recent years the south west and Irish sea has seen a better recruitment of cod and have warmer water than the central North Sea (Influence of Gulf Stream).
Without going into the technological arena, the North Atlantic Oscillation (NAO) plays an important role in the weather fronts that drive sea temperatures in European waters. At the present moment we are in a positive phase, which means milder winter weather patterns, but possibly more stormier than would have been the case in negative phases (the last one in the late 70\'s and early 80\'s).
So what does this mean, in the positive phase we have a stronger subtropical high and a deeper Iceland low, which explains why we keep getting continuous low pressures coming from the northern Atlantic and high pressures dominating (frustratingly) below the UK in the Med.
But moving swiftly on, these persistant streams of warm water may dilute the effects of colder water rising from deeper water (up welling) further reducing the algal blooms vital to fish larvae (anyone noticed a significant reduction or late arrival of jellyfish in recent years), the jellyfish are dependant on algal and zooplankton blooms.
If algal blooms are not as abundant (lack of nutrients), zooplankton may be less viable (reduced fish larvae = less recruitment to the adult stock). If you take the case of the cod, overfishing has had a significant effect, stocks are dominated by sub-adult fish producing less viable eggs, include a shift in the distribution of the cod larvae\'s favoured food (copepods-Calanus finmarchicus) and very quickly the stocks are in trouble. If only a small minority are growing to juvenile size, and they are being hammered not only by the demersal trawlers, but also the industrial fishers are taking their toll, then its difficult to see where the stocks can recover. But its not all doom and gloom, the North Sea is not comprised of one big stock, there are many small individual stocks throughout the area, some of these may be doing better than others. There are indications that those fish further off the Yorkshire coast (an individual migratory stock) are in relatively good shape in comparrison to other stocks, but this doesn\'t mean that they are abundant.
So what does it mean and how does it tie into the potential increase in Wrasse, well in certain areas/regions there may be a lack of cod, as a result other species will fill the predatory niche, this may also produce an increase in prey item availability. Along the Yorkshire coast we\'ve seen over the last 5 years a massive explosion of velvet swimming crab, migrating south, the cod formerly fed on them and held their expansion in check, however, there is also an apparent decline in green shore crab, with those found inshore (beyond low water) of a large size that rarely if ever peel (maximum size).
It is therefore possible, that the velvet crabs are predating on green shore crab (given their high numbers), also likely brown crab and lobsters, to an extent where shore crabs are not the easily available food source they were. Many fish (especially a wrasse) would struggle to feed on a fully grown and armoured velvet crab, a soft crab is a different option, however, the velvet crab moulting period is not as extensive as that of shore crabs, brown crabs and lobsters.
So in a nutshell!!!!!!
By reducing the cod as a top level predator, the velvet crab has been able to increase its population dramatically.
The velvet crab needs to feed and it does so by feeding on other crustacea (shore crabs, brown crabs and lobsters), the density of velvets could very quickly reduce the viability of some species through predation (shore crab), therefore suppressing food availability to higher species in the food chain.
A consequence of this is that the velvets do become prey to higher trophic levels but because of their varocious nature, this is only during a short window when they are peeling. Once the peeling stage is complete, food availability becomes reduced, leaving only resident species like wrasse in the vicinity, whilst transient migratory species (cod) continue to forage out of area.
Therefore, whilst the venue may have produced cod this may have been during times aplenty, once the availability of food is gone, so have the fish, resident species like wrasse are the only realistic species left and they will not turn their noses up at a juicy crab after weeks of sucking on limpets.
So there it is.
Is that the definitive answer? who knows.
Is it viable? certainly.
Does it make sense? probably
If you\'ve managed to get through this then you\'ve done well, take a cold shower and have a beer and we\'ll make marine scientists out of anglers yet.
Cheers
Doc.
