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?"
To answer this question, we need to ask:
- What is sustainability?
- What do we mean by a population of fish?
- What factors affect sustainability (breeding, environmental drivers)?
- How do we measure the influence of these factors on fish stocks (eg biomass, age/size structure)?
- How do we make sure we stay at sustainable levels?
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. Even though the size of a group of fish may change from year to year, 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 year after year.
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 a group of fish to manage, which must be a group of fish that breed.
Fish must be the same species in order to breed and produce young. But even within a single species, there may be separate groups that don't breed with each other.
Fishery managers need to focus in on the group within a group that interbreed. This is referred to as a single fish stock. 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
- Central East Coast Queensland
- Northern Gulf of Carpentaria (Qld)
- Southern Gulf of Carpentaria (Qld).
A fish stock is the correct unit for managers to focus on when they ask questions about sustainability.
3. What factors affect sustainability?
Fishery managers need to know if a group of fish is able to replenish itself year after year.
We need to look at what makes a fish stock able to breed and replenish itself. A number of factors contribute to the ability of fish to reproduce:
(a) Enough fish in the right age classes (population structure)
- Sufficient numbers of females of the right age to breed
- Sufficient numbers of males of the right age to breed
- Sufficient numbers of juvenile fish surviving
(b) Suitable environment
- Safe refuges for eggs and juvenile fish (eg. mangrove wetlands and other fish habitats)
- The right breeding conditions (eg. river flows, temperature)
Many Queensland fish have complicated life cycles. Some change sex at a certain size. Some fish begin their life in freshwater but move to salt water when they get older, and vice versa. These unique characteristics must be taken into account.
Scientists monitor the number of females and males, their size, and age, and the number of juvenile fish.
Environmental factors are also important, such as nursery habitat, and prompts for breeding, such as river flow and temperature.
4. How do we measure the influence of these factors on fish stocks?
When looking at the amount of fish that are available, scientists concentrate on measuring the biomass of a fish stock.
'Biomass' means the estimated quantity or weight of biological material.
To ensure a fish stock is sustainable, we look at how much biomass their is, compared to how much would have been there, prior to fishing commencing. This is sometimes referred to as virgin biomass or unfished biomass. It is an estimate only.
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 what is 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 the combination of rain and temperature and various other factors. The next year may be a 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 Biomass: maximum sustainable yield.
The level of biomass required for optimal fishing efficiency is called Biomass: maximum economic yield.
A good 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 becomes increasingly hard to catch them. 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% 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, 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 consider the current state of a fish stock, based on the measurements above, and apply controls on fishing as required to reach fishery targets help ensure fisheries are managed sustainably by setting out what actions should be taken, depending on how the stock is tracking against benchmarks like the ones below.
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 provides examples of controls on fishing that may be applied to address various types of sustainability factors.
Possible fishing controls
Sufficient numbers of fish
Limits on take (bag limits, catch quota) or effort controls
Sufficient numbers of females and males of the right age
Minimum/maximum size limits (to protect breeders)
Limits on take of females
The right conditions for breeding
Protection of nursery habitat such as mangroves)
Safe refuges for eggs and juvenile fish
Spawning closures (areas, seasons)
Where can I find more information?
To keep up to date with the Sustainable Fisheries Strategy, visit our website - http://www.daf.qld.gov.au/fisheries