Most commercial fisheries supplying fish for human consumption will seek to capture species which are economically valuable and in demand by consumers.
However, the catch often includes fish, benthos, macrophytes, and occasionally marine mammals and seabirds which are unsuitable for consumption and/or have little or no market value. Usually, this material is sorted from the catch and discarded at sea.
Regarding the fish components of the catch, modern-day discarding decisions made by fishers depend on a complex interaction between legal and practical constraints and economic trade-offs.
Legal issues include minimum landing sizes and quota restrictions on quantities of each species that can be landed, the extent of enforcement, and the penalties for infringement.
Practical constraints are factors such as the amount of catch that can be accommodated aboard the vessel, whilst economic trade-offs include market values of each species relative to their abundance in catches.
However, regardless of the reason for discarding, the majority of rejected fish are already dead or fatally damaged as a result of the capture process, adding unintended mortality to both target and non-target species.
For this reason discarding is widely regarded as being wasteful and contrary to ethical principles of exploiting living resources.
More data needed on fish discard rates
Information on quantities of fish which are discarded is important for a variety of reasons.
First, if discards constitute more than a trivial fraction of catches, then stock assessments based solely on landed quantities of fish will result in biased estimates of mortality rate, in turn leading to erroneous perceptions of sustainable exploitation rates.
Secondly, devising regulations to curtail discarding requires good knowledge of contemporary discarding practices so that precise forecasts of catches can be made.
Finally, the biomass embodied in discarded fish is not lost to the ecosystem, but constitutes a potentially important food source for scavenging and opportunistic-feeding species including other fish, benthos, birds and mammals.
The ecosystem impacts of regulations to reduce discards cannot be determined in the absence of comprehensive information on quantities involved.
Monitoring of the quantities of fish landed at ports is, in principle, a straightforward task since the fish are generally sorted and weighed prior to sale.
However, gathering information on the quantity and composition of fish discarded at sea is extremely demanding, requiring observers aboard individual vessels to collect data without interrupting the normal fishing operations.
Typically, observers can be placed on only a small proportion of vessels and fishing trips.
In addition, the cooperation of vessel skippers and crew may be voluntary, and fishers may modify their behaviour in the presence of observers, leading to potential bias in the observed discarding rates.
Problems with estimating discard rates on a regional level
Expanding data from observer samples, which are thinly spread across a range of fishing methods, vessel sizes, regions and times of year, to produce regional estimates of total quantities of fish discarded, represents a formidable statistical challenge.
There are a number of approaches, but most rely on estimating the mean proportion of catch weight which is discarded in the sampled fraction of the fleet.
Then, assuming that this proportion applies uniformly across the fleet as a whole, and given the fleet total weight of fish landed, the total weight discarded is calculated.
For heavily discarded species, estimates of the total weight discarded are highly sensitive to uncertainty in the proportion discarded, making the estimation of total quantities discarded problematic.
In the North Sea, where observer-based fisheries sampling has been carried out by some nations for more than 30 years, credible time-series of species-specific, regional-scale annual discards integrated across the full range of fleets, have been compiled only for the five major targeted demersal species (cod, haddock, whiting, plaice and sole).
Data on all the other species captured by fisheries in the North Sea are largely insufficient or inaccessible for the purposes of estimating annual weights discarded, for the years prior to the introduction of EU standardised data collection regulations in 2000.
Various methods have been proposed for improving the expansion from observer data to regional discard estimates—for example, the use of selectivity data to reconstruct discards from landings or as part of stock assessment procedures, the use of covariates and techniques for identifying outliers among discard samples and improving the precision of regional averages.
However, all these approaches still rely on the problematic expansion from landings to catch using observed data on the proportion discarded.
Only limited further information on the past and current extent of overall discarding can be gleaned by such approaches. Moreover, these methods will become unworkable as jurisdictions begin to introduce legal landing obligations on fisheries.
As soon as discarding of any species becomes an illegal activity, placing observers aboard fishing vessels to gather data on species which can still be legally discarded will become impossible, since the observer cannot any longer be impartial and the relationship with the vessel skipper will be compromised.
A fresh approach is needed to draw-in additional data
Incorporating additional information on the length-composition of landings ought to provide a basis for a more refined estimate of discards given knowledge of length-dependent retention probabilities.
Landed length compositions are monitored at ports typically only for a few major targeted species. Unfortunately these data are often difficult to access since the standard reporting for input to stock assessments is in the form of derived age-compositions.
Reconstruction of discard estimates from age-based landings data requires some problematic assumptions about discard rates in relation to age rather than size, or approximate reconstructions of landed length-compositions from age-compositions.
In any case, there are no data on length distributions for the majority of landed species; only the total landed weights.
New statistical models
This study has addressed the problem of estimating discards from a different perspective by developing a statistical model to hind-cast time series of the quantities of catch discarded for individual species.
The model uses data on landed weights and length distributions recorded from scientific research vessel trawl surveys.
In addition to providing a comprehensive assessment of discarding rates and quantities, incorporating poorly sampled species, the method offers the prospect of estimating discard rates in a future legal framework where deploying observers aboard fishing vessels becomes impractical.
As a case-study, the model is applied to the North Sea over the period 1978-2011, and shows a long-term decline in the overall quantity of fish discarded, but an increase in the proportion of catch which is thrown away.
The composition of discarded catch has shifted from predominantly (~80%) roundfish, to >50% flatfish.
Undersized plaice constitute the largest single fraction of discards, unchanged from the beginning of the 20th century.
Overall, around 60% of discarded fish are rejected on the basis of size rather than for reasons of species value or quota restrictions.
The analysis shows that much more information can be gained on discarding by utilising additional sources of data rather than relying solely on information gathered by observers.
In addition, it is clear that reducing fishing intensity and rebuilding stocks is likely to be more effective at reducing discards in the long term, than any technical legislation to outlaw the practice in the short term.
