Managing fisheries for sustainability

Sustainably managing our fisheries

Sustainability is the central goal of the Queensland Sustainable Fisheries Strategy.

The most important question for fisheries managers is "how many fish can be taken before fishing becomes unsustainable?"

Our film Sustainably managing our fisheries explains how we do that.

1. What is sustainability?

A sustainable population simply means a group of fish that is able to sustain itself year after year.

Fish reproduce and replenish their numbers to replace those lost to predators, disease, or aging. If the group of fish can replenish its numbers to a similar level in future generations, it is considered sustainable.

But if too many fish are lost, for example from fishing pressure, that group of fish may not be able to replenish its numbers.

Figure 1: Example of changes to a population of fish in response to fishing pressure over time.

2. How do we manage for sustainability?

We first need to define what we mean by a population of fish – it's a group of fish that breed.

Fish must be the same species in order to breed and produce young. This is referred to as a single fish stock. There are some exceptions, for example, barramundi is a single species of fish (Lates calcarifer), but in Queensland we identify 6 different barramundi stocks:

• Princess Charlotte Bay
• North East Coast Queensland
• Mackay
• Central East Coast Queensland
• Northern Gulf of Carpentaria (Qld)
• Southern Gulf of Carpentaria (Qld).

3. What factors affect sustainability?

We need to look at what makes a fish stock able to breed and replenish itself. Scientists monitor the number of females and males, their size and age and the number of juvenile fish.

These environmental factors are also important:

• nursery habitat (refuges for eggs and juvenile fish in areas of mangrove wetlands and other fish habitats)
• prompts for breeding (e.g. river flow, temperature).

4. How do we measure the influence of these factors on fish stocks?

To ensure a fish stock is sustainable, we estimate at how much biomass there is compared to how much would have been there prior to fishing commencing. This is sometimes referred to as virgin biomass or unfished biomass.

'Biomass' means the estimated quantity or weight of biological material. Scientists concentrate on measuring the biomass of a fish stock.

How do we estimate biomass?

Obviously, we can't count or weigh all the fish in the sea. The total biomass of a stock is estimated by fisheries scientists, using models based on information such as:

• fisheries-independent sampling
• catch and effort data
• research on related fish stocks

What is the right level of biomass?

To know the right level of biomass, we need to choose a benchmark, or reference point. This is difficult because fish stocks change year to year - some years are bumper years for a particular species, because of rain, temperature and other factors - other years may be lean year for the same species.

The most important goal is sustainability. Fish stocks must be healthy enough to replenish themselves. Usually, a fish stock needs to be about 30-40% of unfished biomass to remain sustainable. The minimum biomass required for sustainability is known as maximum sustainable yield.

The level of biomass required for optimal fishing efficiency is called maximum economic yield.

A better benchmark is the level of stock needed to optimise efficient fishing. Putting more effort into fishing doesn't always mean catching more fish. Up to a certain point, if you put in more effort, you'll get higher rewards. But when there aren't enough fish available, it will be harder to catch them, so putting in more effort doesn't necessarily mean higher rewards.

The ideal is to get the best rate of return on your effort (the most bang for your buck). This ideal rate of return is possible when larger amounts of fish are available. In general, this requires about 60 per cent of unfished biomass.

Figure 2: Example of management responses that may be applied by harvest strategies, depending on the health of fish stocks (measured by biomass)

Population structure (length, age, sex ratio, maturity)

It's not enough to know how many fish there are, we also need to know if enough breeding adults are around, and if a large enough number of juveniles are surviving.

If fish become mature at a smaller size than usual, it can be an indication that the population is under stress and trying harder than usual to replenish itself. To check for changes such as this, fisheries managers regularly monitor the length, age, sex ratio and size at maturity of fish stocks.

5. So how do we make sure we stay at sustainable levels?

Harvest strategies help ensure fisheries are managed sustainably by setting out what action should be taken, depending on how the stock is tracking against benchmarks.

Harvest strategies also define how these fishing controls may change in response to the performance of the fish stock.

A number of controls on fishing can be applied, like changing quotas and bag limits, or other actions like changing size limits or seasonal closures.

The table below shows examples of controls that may be applied to address various sustainability factors.

Where can I find more information?

Visit the Sustainable Fisheries Strategy page for more information.