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Counting our fish for a sustainable future

Wednesday 12 October 2016


As fisheries scientists we gather a lot of information about fish. We collect and analyse data on how many of each species are caught, where, how often, what size or age they are, and how their abundance changes over time.

All this information is fed into population models, statistical approaches that enable us to understand complex interactions, such as how varying levels of catch or birth rates of young fish affect population size. They also allow us to predict what is likely to happen in the future under different catch scenarios. We are looking to define the most desirable stock status.

We know that in an unfished state, fish populations exist at the carrying capacity of the system they live in and mostly consist of older, slow growing individuals that are not very productive, as is the case for a mature forest. Younger populations with more fast-growing fish, allow for higher levels of production and sustainable harvests.

However, if abundance gets too low, the sustainability of a fish stock may be at risk. In New Zealand, we regard a fishery with an abundance below 20 per cent of the original unfished stock size as overfished and in need of a formal rebuilding plan.

Below 10 per cent it is regarded as collapsed and may need to be closed to ensure its recovery. Fisheries management programmes aim to keep abundance well above the 20 per cent level and usually within a target range of 35 to 50 per cent.

CASAL population models

NIWA assesses the status of many of the fish stocks around New Zealand as part of its work for the Ministry for Primary Industries (MPI). CASAL, the main assessment model we use, was developed by NIWA about 15 years ago and uses statistical integrated analysis methods to assess the status of individual species.

There is a wide range of data that goes into a typical stock assessment. Scientists use data on catch levels; biological information such as growth rates, age at sexual maturity, and natural mortality; changes over time in age and length structure of the population, trends in commercial catch rates; and scientific survey abundance estimates.

Senior fisheries modeller Dr Ian Doonan says while CASAL was ahead of its time when it was developed more than a decade ago, it has been overtaken by recent developments in assessment methods. “Casal2, is the newly-released and highly augmented next generation version of this assessment model that will allow us to assess multiple species and stocks in multiple areas simultaneously.

We can now also incorporate a range of ecosystem dynamics into the analyses, including predator-prey interactions, finer spatial scale processes, and environmental change."

Hoki population trends

Hoki is one of New Zealand’s largest and most valuable fisheries, with a current annual catch of about 150,000 tonnes and an export value last year of more than $200 million. The fishery was developed in the mid-1980s when the Total Allowable Commercial Catch (TACC) was increased from 60,000 to 250,000 tonnes.

There are two stocks: a western stock which spawns off the west coast of the South Island, and an eastern stock which spawns in Cook Strait. Key information going into the population model includes trawl surveys in the two main non-spawning areas and acoustic surveys in the two main spawning areas, and catch-at-age from the trawl surveys and the four main commercial fishing grounds.

One of the trawl surveys, on the Chatham Rise, also provides valuable estimates of one and two-year-old hoki that are not yet available to the fisheries. The population model produces estimates of annual population size over time for both stocks.

The initially larger western stock experienced an extended seven year period of poor recruitment from 1995 to 2001, which may have been related to warmer sea temperatures during this period. In conjunction with fishing levels at the time, this caused a decline in abundance in the mid-2000s. Significant reductions in the TACC (from 250,000 tonnes down to 90,000 tonnes) and catches and better birth rates since 2002 have resulted in a rapid rebuild of the population so that it is now above the upper end of the management target level.

The current TACC has increased to 150,000 tonnes. The future also looks promising with estimated high birth rates of fish spawned in 2011 and 2014. The hoki fishery is a good example of how wild native fisheries can fluctuate due to both environmental and fisheries influences, how science can monitor and model these changes and, in combination with responsive management, how this results in productive and sustainable fisheries.

This fishery was the first fishery in New Zealand, and the first white-fish fishery in the world, to achieve the prestigious Marine Stewardship Council (MSC) certification for sustainability, in 2001. But what about smaller or less valuable fisheries for which relevant data may be limited?

Internationally, there has been increased attention on developing methods to assess and manage fisheries that may have limited data, or resources, to carry out detailed statistical modelling. For example, we may have no information on abundance or an incomplete picture of how much has been caught.

Nevertheless, estimating the population status of these species is still possible, even though it may be less certain. When information is limited, we need to make stronger assumptions about how biological populations function and we often need to use much simpler models.

For example, a CASAL model may include information on maturity, mortality, and growth, whereas a simpler model may combine these into a single parameter called ‘productivity’.

NIWA is currently developing a range of approaches to provide assessments for these type of situations.

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