Sunday, June 28, 2015
MERE BIOMASS ISN'T ENOUGH
By any conventional measure, the Atlantic Herring stock is in good shape. Not only is the stock neither overfished nor experiencing overfishing, but fishing mortality is well below the threshold and biomass is more than double the target.
If we looked at the stock from a pure biomass standpoint, it is exceedingly healthy, and even considering all of the factors that go into a single-species model, the herring are doing well.
But the problem, of course, is that we’re using a single-species model, and when you’re dealing with a forage fish such as herring, which is preyed upon by a host of other species, single-species models don’t tell the whole story.
That’s why a lot of people might have been surprised recently, when they read that bluefin tuna in the Gulf of Maine, which feed mostly on herring, are not eating enough to get into prime condition needed for the long migrations south to spawning grounds in the Gulf of Mexico.
That seeming contradiction puzzled scientists, too, when they noticed it some time ago. They set out to understand why, and in a recent paper, entitled “The paradox of the pelagics: why bluefin tuna can go hungry in a sea of plenty,” provided their explanation.
It turns out that in order to obtain proper nutrition, bluefin tuna need to feed on older, larger herring; an abundance of small herring don’t provide enough nutrients to allow the fish to build up the fat reserves needed for their southern migration.
That lack of fat reserves has real implications for the health of the bluefin population. The paper’s authors note that
“A reduction in [fat] stores can have profound effects on fish life history…Faced with reduced energy, spawning age fish can reduce growth and reallocate more reserves to gonadal investment. Such a strategy can maintain fecundity in times of reduced condition, but may also contribute to higher rates of post-spawning mortality. Alternatively, fish may skip spawning in times of reduced condition or change reproductive output (fewer eggs, lower quality) for females. Given the effect of total [fat] content on egg production, the influence of prey quality on predator life history parameters and the relationship between bluefin tuna weight and fecundity, a reduction in bluefin tuna condition on the foraging grounds could have affected reproductive output…”
That, in turn, may help to resolve one of the big debates in bluefin tuna management—why the harvest restrictions imposed to date have not allowed the stock to rebuild to 1970s levels and, whether environmental conditions are such that the western stock of bluefin is not as productive—that is, cannot produce as many young fish—as it was a half-century or so ago.
Although no firm conclusions can be drawn from the data currently available, the paper notes that
“…total allowable catches have been in line with scientific advice for over 2 decades. If fishing mortality were the sole factor limiting population recovery, levels of fishing mortality set by management should have allowed this stock to rebuild much faster than currently observed, even for a species with long generation times like bluefin tuna. A slow recovery may be related to fishing mortality set too high, despite the scientific advice, or could be the results of environmental factors affecting life history (growth, reproduction, migration) or synergistic effects between the two…”
But if the lack of large herring in the population is causing the bluefin tuna stock to be less productive, and thus preventing the stock from rebuilding to historical levels, there is no clear biological path to remedy the problem.
First, the reason why there aren’t enough large herring isn’t at all clear.
As the paper’s authors note,
“Studies…on George’s Bank suggest the shift to a smaller herring community (as indicated in the negative relationship between herring size and abundance) is a direct result of density dependent growth.”
In other words, when herring are abundant, they may grow more slowly and perhaps never grow as large as they would if there were fewer fish.
“Increased fishing pressure can reduce density dependence and potentially lead to more positive weight anomalies for these small pelagics. However, intense fishing also reduces the abundance of older, larger fish, and management strategies that produce a high abundance of faster growing but smaller individuals would shift the size spectrum towards smaller fish.”
In that case, anything gained by reducing the abundance of herring so that fish might grow larger could be lost by a fishing regimen that harvests most individuals before they could attain such large size.
Second, while bluefin appear to benefit when large herring are available, even if herring abundance is down, other predators need an abundance of smaller individuals to thrive.
Humpback whales, for example, feed by rising through schools of herring and engulfing many fish at one time. They get greater benefit from abundance than from the size of the individual fish.
And that, in the end, is what makes managing forage fish so challenging, and so different from managing species at higher trophic levels. Single-species management, which emphasizes sustainable harvest, does not and cannot adequately account for the needs of all of the various predators that also rely on the targeted resource.
As the authors of “The paradox of the pelagics” note,
“indicators of size structure and body condition should be considered when managing prey species to ensure higher predator fitness. The challenge will be implementing such a strategy within the context of current assessments and management models and determining the relative importance of prey abundance and size for a range of predators with different foraging strategies and metabolic needs.”
That is true not only for Atlantic herring, but also for such species as menhaden, which serve as forage for a very wide array of predators.
And it illustrates why a very simplistic management approach, which merely looks at abundance and fishing mortality to determine whether there will be enough fish around to catch in the future, is not nearly good enough to manage forage species.
When we talk about managing forage fish, biomass is only one bit of data that’s needed to make decisions on harvest. How that biomass is structured, and how it is used by a myriad of predators, must also be known before decisions are made.