As a silvery sphere of herring schools through the open North Sea spatially uninterrupted by other species, its whereabouts is picked up clearly by the colour echo sounder of the fishing vessel pursuing it. The ship in question has set out for that particular species and, as it deploys its gear and subsequently hoists up its catch, it can do so safe in the knowledge that the net will contain next to no other species of fish as bycatch.
In fisheries assessment terms, this could be called a 'clean' or 'single-stock' fishing operation. Whether herring or other pelagic fish species, a ship's haul in this case will fall in line with the Total Allowable Catch (TAC) for that species and the allocation of that given to the vessel.
Yet the picture stretches beyond that. Whilst individual fishermen head out to sea to search for grounds which yield the fish they are permitted to land, they can actually end up filling their nets with a range of different fish species that share seawater space and prey resources with each other.
Catching certain species that are supposed to be left in the sea, and thus not covered by the TACs, is sometimes unavoidable, especially with the disparate size (and respective price and interest) shown by different stocks. This predominantly occurs in demersal zones, as those in the North Sea, where deeper-water species like cod and haddock exist alongside bottom-dwelling flatfish such as plaice and sole. It is this marine intermingling of species coupled with the likelihood of a mixed bag of fish in the net that characterizes mixed fisheries.
Whilst mixed fisheries are defined by the harvesting of multiple types of fish by vessels using the same kind of gear, fleets – numbering 37 and representing eight countries as part of ICES model of North Sea demersal fisheries – using a range of gear such as otter trawls, demersal seines, and gillnets are all active in the North Sea.
Some of this gear can be more effective in targeting the chosen species than others; indeed, certain sorts are more selective than others, resulting in the assortment of fish potentially found in the net. This, in what is one of the challenges for mixed fisheries management, can lead to bycatch and discards as well as jeopardizing the recovery of weak stocks.
The crux of producing advice for mixed fisheries lies in the technical interactions between the resource – the fish stocks or species being targeted – and the activity of fishing itself. A technical interaction here then means the fish interact through ending up in the same fishing net. This concept is in contrast, though of equal advisory importance, to multi-species advice, which revolves around how fish biologically interrelate, like through predation. Cod on Capelin, for example.
The job of mapping the complexities of what happens at sea for ICES mixed fisheries advice – and the reality that actual catches don't always mirror advice – rests with the Working Group on Mixed Fisheries Advice for the North Sea (WGMIXFISH-NS), an ensemble which held its annual meeting at ICES Secretariat at the end of May.
Since 2002, when the limitations of the traditional, single-species approach to advice were brought to light by the conflicting states of demersal North Sea fish stocks, ICES and its expert groups have acted to evolve a fishery-based alternative. Born from a string of mixed fisheries modelling workshops and at the request of ICES clients, WGMIXFISH took over to supply advice for mixed fisheries, taking the advice drawn up on various single stocks as the basis of its work.
The North Sea has been a trailblazer in this way; such advice for other ecoregions – the Celtic Seas, Bay of Biscay, and Iberia – is still in the development pipeline (with WGMIXFISH-METH looking at the methodology for bringing it in).
One of WGMIXFISH's members' primary tasks is to devise forecasts for mixed fisheries based on the individual species advice on North Sea cod, haddock, whiting, saithe, plaice, sole, and Nephrops (Norway lobster). For these projections, the group employs several different approaches – including each stock's long-term management plan, if one is in place, and Maximum Sustainable Yield (MSY) – through which it formulates five separate scenarios for the ecoregion, each of which helps predict a number of landings per stock.
Unlike single stock advice, there is no single recommendation, so these scenarios assume the roles of conceivable catch options. Since it isn't possible to achieve all management objectives simultaneously, this method aims at presenting sets of trade-offs for whatever choices are made at sea.
Adopting a modelling tool known as Fcube (Fleet and Fisheries Forecast), the group processes single-stock data into the following five scenarios: 'min', where fleets stop fishing when their first quote is exhausted; 'max' when their last quota is exhausted; 'cod', when the cod quota is exhausted; 'status quo effort' where effort is equal to most recent year; and 'effort management', where effort is reduced according to regulations. In the third outcome, cod is commonly, though not always, designated as a 'choke species': a low-quota species that would lead to vessels having to cease fishing activity altogether.
In terms of ICES advice, these scenarios are then accompanied by an analysis of the implications of mixed fisheries under both the fishery's Total Allowable Catches (TACs) and effort regime.
The evaluation and scenarios formed part of ICES advice for the EU on Bay of Biscay, Celtic Sea, and North Sea fish stocks, when it was issued on 30 June.
Fishermen hoisting their catch; © National Marine Fisheries Research Institute (NMFRI), Poland