Sustainable Fishing and Maximum Sustainable Yield
"Cod” wrote Smith Homans, and J Smith Homans back in 1858, is “a species too well known to require any description. It is amazingly prolific. Leeuwenhoek counted 9,384,000 eggs in a cod-fish of middling size – a number that will baffle all the efforts of man to exterminate”. But we are a clever, highly-adaptive species, and it didn’t take too long before we were no longer baffled. Fisheries prospered, and then fisheries collapsed. Eighty-five years after the Cyclopaedia of Commerce and Commercial Navigation was printed, a new mantra for fisheries was voiced by Michael Graham: “Fisheries that are unlimited become unprofitable”.
Today we are very much aware of just how rapidly we can deplete any fish stock, and the socio-economic impacts on fishers, and dependent trades and communities. Today, step into the Exclusive Economic Zone of America, and you will find commercial fisheries managers striving to find how to maximise extraction from a stock without causing its collapse – the search for the ‘optimum catch’...
The idea of an optimum catch is not new. Its introduction into fisheries science began in the early 20th century, most notably during the 1930s through the work of fishery scientists like Hjort, Ottersadf, Jahn, Graham, Russell, Beverton and Holt. It wasn’t until the late 1940s/50s that the concept of ‘maximum sustainable yield’ (MSY) became a part of fishery management.
The principle of MSY is almost beautifully simplistic. For many marine species, as Leeuwenhoek observed, an individual can produce an extraordinary number of eggs. How many go on to survive and reach maturity depends on a whole host of factors – like the availability of food: the more food available, the greater the chance of survival. As individuals procreate, population growth will continue until there isn’t enough food available for all those hungry mouths (the carrying capacity of the environment). If we were to fish stocks down to levels where population growth was greatest, then, in theory at least, a fishery would be sustainable because by removing individuals surplus to growth (preferably the ones that have already had a chance to spawn) population growth would kick in, naturally replacing whatever we took out. But we have to be careful. Take too many individuals out of the population and growth slows and even declines if we take out more than can be replaced through reproduction.
This optimal point – the MSY – was thought to be around 50% of the stock’s unexploited biomass, though this figure is now somewhat questionable. These days there are several variations of MSY, such as SSBMSY – which focuses on the sexually mature biomass of a stock able to produce MSY – or FMSY – the level of fishing mortality required to keep a stock at MSY, after natural mortality. If an MSY point is chosen as the benchmark on which to determine the status of a fishery, then fishing beyond MSY means you are overexploiting the stock. Fish below it and you are not yet fully exploiting the stock.
“Essentially, all models are wrong, but some are useful,” statistician George Box told us. So is MSY useful? To start to answer that, perhaps the best place to start is with the 1949 United States Policy on High Seas Fisheries. Set in motion by Wilbert Champan, an ichthyologist who believed that American fisheries could – and should – set their sights on the bounty of fish occupying the Pacific Ocean, the policy set out America’s right to claim parts of the then high seas for resource extraction – and the “conservation and protection of fishery resources”. MSY was the keystone for ensuring optimal fishery extraction, whilst ensuring their conservation and protection.
Treaties with other nations that swiftly followed saw the spread of MSY. When, in 1955, the International Law of the Sea was introduced, MSY became an internationally recognised objective, echoed later in the Convention on Biodiversity, and the World Summit on Sustainable Development. But there was a catch. When MSY was brought into US policy, it was severely lacking in empirical evidence. The question was not if MSY was accurate, and thus suitable as a policy objective, but how to make MSY work for policy.
Perhaps the most notable issue with MSY is the assumption that a stock’s productivity is purely a function of its spawning biomass. We now know that stock structure – its age and size composition, and even its spatial distribution – are factors important to productivity. In particular, we have found that in some cases not only are smaller and/or younger individuals less fecund than their larger/older counterparts, but that the offspring from larger/older individuals have greater survivability. Both desire and policy have focused effort on larger/older fish, truncating stocks, impacting their composition and thus productivity. Furthermore, the size and makeup of any stock is not constant – even without human predation. Natural mortality for any given stock can depend on all sorts of factors – like the size of a population that may predate on it or the environmental conditions during developmental stages.
This leads us to another problem. In order to calculate the MSY, we need to know what the unexploited biomass of that stock is. Biomass is not constant, and thus the MSY point cannot be static in perpetuity. For managers, this presents a challenge in terms of data and resources – particularly for those who lack financial resources to collect data that is so crucial to the development and maintenance of sustainable fisheries.
Rightly or wrongly, catch per unit effort (CPUE) is often used as a proxy for abundance, which is then in turn (ideally in conjunction with more detailed stock assessments) used by managers to calculate where in relation to the MSY a fishery is. Collecting and analysing accurate and sufficient data does not come cheap. Collecting and analysing data for more detailed stock assessments is even more costly. Even when all this data are available, accurately estimating stock size at any given point and determining when a fishery has hit MSY is no simple task.
It is also worth noting that Chapman believed that once the alarm bells of overexploitation were ringing, governments would promptly curtail fishing effort. It was presumed that upon reducing – or even ceasing exploitation, a stock would rebuild back to its unexploited level, at which point fishing could recommence. History tells us a very different story. The International Commission for the Conservation of Atlantic Tuna (ICCAT) have become somewhat well known for ignoring scientifically recommended catch levels for Atlantic bluefin tuna; the 1990 moratoriums on cod fishing in the Northwest Atlantic have not resulted in rebuilding to levels sufficient to lift the ban.
No species is an island, and when we fish for one species, directly and indirectly, we impact on another. The failure of MSY to recognise stocks as more than isolated entities has drawn substantial criticism.
The first issue relates to the idea of harvesting only the fish that are “surplus” to production. What appears surplus to us is actually food for something else. Declines in prey can lead to declines in predators, and fishing predators can lead to increases in prey (trophic cascades). When stocks of the ‘forage fish’ menhaden in the Eastern Atlantic were overexploited in the 1870s, fishermen rioted because the decline of menhaden, they believed, had led to the decline in other species – like cod – that predate menhaden. To this day, the menhaden fishery is a contentious one, and concerns about food-web upheavals are ever-present. Declines in one species can also allow another to undergo ‘competitive release’, because competition for shared resources has become more abundant.
One of the contributing factors to the failure of Cod in the Northwest Atlantic to make as large a comeback as expected could be partially due to the increase in Atlantic herring that occurred as cod declined. Unfortunately for the cod, herring are also quite partial to cod eggs and larvae. The ecosystem impacts of harvesting ‘surplus’ individuals can go beyond the marine environment and the terrestrial realms, such as the temperate rainforests of British Columbia, whose rich and diverse ecosystem depends on the annual migration of salmon that spawn and die in its rivers, providing vital nutrients.
The second issue pertains more to interspecific interactions – the tendency for different species to be present in the same location at the same time, giving rise to what are known as mixed fisheries. Mixed fisheries are a headache for the application of MSY, because each stock in a mixed fishery probably has a different MSY. Unfortunately, fishing gear is not selective enough to fish one species to MSY whilst ensuring another is not fished beyond it.
Controversy surrounding MSY is nothing new, with arguably the most famous rebuttal coming from Philip Larkin in 1977, whose epitaph for the concept of MSY began with: “Here lies the concept of MSY. It advocated yields too high”. Like all models, MSY is wrong, but it is not without its use. Key to sustainable fisheries is the incorporation of both scientific and management uncertainty. Such policies recognise that in reality the only thing we can control is our own actions – what we remove from the ocean, how much of it we remove (FMSY), and the methods that we use to do so. MSY can form a minimum reference point for stock levels as opposed to a target for extraction; for example, requiring stocks to be at a level that can produce – not at – MSY (SSBMSY).
In a mixed fishery context, whilst it is close to impossible for all species to be at MSY at the same time, it is feasible to have all stocks above MSY at the same time. In this scenario, a mixed fishery would effectively have an MSY of the most sensitive stock – regardless of the status of the other species within it. Whilst keeping a stock to a minimum reference point does not guarantee a long-term sustainable fishery, the stock’s health would probably be in better shape than if no reference point was used at all. There are examples of stocks managed with MSY objectives that are considered to be in ‘good health’. These include ICES-managed herring in the Celtic Sea which (as at July 2013) has a spawning stock biomass above management targets. How appropriate such management targets are is another debate.
MSY is not perfect, being arguably too simplistic to be applied to a complex ecosystem. Suggestions have been made to use other reference points as targets, either alongside or instead of MSY, including the Maximum Economic Yield (MEY) and Ecologically Sustainable Yield (ESY). Just like MSY, none of these reference points is perfect but, if used appropriately, could prove useful tools in the implementation of more sustainable fishing practices.
Fishery targets are just one of many strings in the bow needed to work towards the somewhat elusive goal of sustainable fisheries. These include reducing habitat damage and by-catch, altering fishery behaviour that can truncate age and size distributions of stocks, minimising the impairment of ecosystem functioning, and remembering that we are not the only species that relies on ocean resources. Good governance that is dynamic, transparent, inclusionary with stakeholders, and includes buffers for scientific and management uncertainty is also important, as is reducing the at times seemingly opposite of demand and poverty.
This article originally appeared in Marine Scientist in August 2014. Marine Scientist is only available in print format.
