Fisheries Science

Fisheries management draws on fisheries science in order to find ways to protect fishery resources so that sustainable exploitation is possible. Fisheries science is the academic discipline of managing and understanding fisheries. It is a multidisciplinary science, which draws on the disciplines of oceanography, marine biology, marine conservation, ecology, population dynamics, economics and management in an attempt to provide an integrated picture of fisheries.

The traditional approach to fisheries science and management has been to focus on a single species, using a stock-recruitment relationship. A more modern fisheries model is the ecosystem-based approach.

Stock Recruitment Model

Female fish and shellfish produce astounding numbers of eggs, giving any population the capacity to increase its density rapidly after a perturbation if conditions are right for the survival of the young. This rapid reproductive rate (r-selected species) allows humans to harvest fish populations and anticipate their recovery. The degree to which a stock may be harvested has historically been determined by the form of the S/R relationship.

The S/R relationship is normally presented graphically as a scatter plot with the number of females in the spawning stock on the abscissa (x-axis) and the number of recruits on the ordinate (y-axis). The spawning stock is defined, normally, as the number of female organisms in the population of reproductive age and able to reproduce in any one year. The recruits are defined as those young who survive to either maturity, or to be captured by the fishery.

An explanation of the Ricker stock-recruitment curve.
Click image to enlarge.

The S/R relationship is normally dome-shaped, facing down. This means that we expect zero or very few recruits when the spawning stock is very low (in other words, the relationship passes through the origin), that we have maximal recruitment for a middling number of spawners, and that recruitment is badly reduced if there are too many mature adults. This latter point is best understood if we realize that adult and immature fish often compete for food, with the larger adults winning this competition. Thus, if there are many adults, survival rates of the young and immature fish will be very low, leading to low recruitment rates.

The replacement line is where stock = recruits. Any recruits above this line are considered to be "in excess" of that required to maintain the population, and can therefore be harvested without impact to the population.

There are two classical mathematical models used to describe the relationship between the stock and the number of recruits. The first is called the Beverton-Holt model, which states that R=E/(E+g*R_max)*R_max, where g is a parameter, R is the number of recruits, E is the egg production (number of females * average egg production). Shortly thereafter, Ricker suggested the following model (now called the Ricker curve): R=R₁*E^-R2*E where R₁ and R₂ are parameters. More recently, Deriso and Schnute have proposed a more general model, which reduces to either of the former models when certain parameters attain some value. Their model is: R=R₁*E*(1-R₂*R₃*3)^1/R3.

These models, and some variants, have been used to manage fish stocks for the past fifty years. In recent years they have come under criticism for a number of reasons, both theoretical and practical. On the theoretical side, they do not account for systematically changing environmental conditions, changes in the water currents or immigration/emigration. The practical problems are that, despite a good theoretical foundation, they have a remarkably poor track record. Many enormous fish stocks have been carefully managed into near-extinction by the use of these models (eg. Atlantic cod, the anchovy, the salmon). Modern management approaches still consider the S/R relationship when formulating their harvesting recommendations, but they are only one of many different approaches used in an integrative manner.

An example of some integrative stock assessment tools is the NOAA Fisheries Toolbox.

The stock synthesis model from the NOAA Fisheries Toolbox.
Click image to enlarge.

Ecosystem-based Model

Ecosystem-based management is an environmental management approach that recognizes the full array of interactions within an ecosystem, including humans, rather than considering single issues, species, or ecosystem services in isolation.

Image from EarthLabs: Oh What a Tangled Web:
Ecosystem-Based Management - see link below.
Click image to enlarge.

Ecosystem-based fishery concepts have existed for some years and have been implemented in a few regions. Some of the guiding principles in ecosystem-based fisheries management are:

1. Keep a perspective that is holistic, risk-adverse and adaptive.
2. Maintain an “old growth” structure in fish populations, since big, old and fat female fish have been shown to be the best spawners, but are also susceptible to overfishing.
3. Characterize and maintain the natural spatial structure of fish stocks, so that management boundaries match natural boundaries in the sea.
4. Monitor and maintain seafloor habitats to make sure fish have food and shelter.
5. Maintain resilient ecosystems that are able to withstand occasional shocks.
6. Identify and maintain critical food-web connections, including predators and forage species.
7. Adapt to ecosystem changes through time, both short-term and on longer cycles of decades or centuries, including global climate change.
8. Account for evolutionary changes caused by fishing, which tends to remove large, older fish.
9. Include the actions of humans and their social and economic systems in all ecological equations.

Ecopath with Ecosim (EwE), is an ecosystem modelling software suite. It was initially a NOAA initiative, but now primarily development takes place at the Fisheries Centre of the University of British Columbia.

EwE has three main components:

1. Ecopath – a static, mass-balanced snapshot of the system
2. Ecosim – a time dynamic simulation module for policy exploration
3. Ecospace – a spatial and temporal dynamic module primarily designed for exploring impact and placement of protected areas

The Ecopath software package can be used to:

1. address ecological questions
2. evaluate ecosystem effects of fishing
3. explore management policy options
4. analyze impact and placement of marine protected areas
5. predict movement and accumulation of contaminants and tracers (Ecotracer)
6. model effect of environmental changes

Click image to enlarge.