How can we prevent the extinction of whitebait?
Waterways
Centre for Freshwater Management
University
of Canterbury
--
A recent NZ Geographic article has stirred up an already
vigorous debate (see the full article here goo.gl/TcWWU8).
“What we call ‘whitebait’ are the
juveniles of five fish species: giant kōkopu, banded kōkopu, shortjaw kōkopu,
īnanga and kōaro. The first is predicted to go extinct by 2020, the second by
2023, and the rest by 2034”.
The key point here is extinction. It refers to the death of the last
individual. If the above predictions were correct this would occur in 3, 6 and
17 years for these species.
The actual 'extinction event' might occur through one of these three ways:
1) all adult fish die before spawning
2) the remaining adult fish spawn but all the eggs die
3) eggs hatch but larvae and juvenile (i.e.whitebait)
mortality causes extinction (or contributes to #1)
Are whitebait really going extinct?
This is a very good question and some important clues relate to what we know about historical trends. Bear in mind that the actual numbers of fish, eggs, etc are very difficult to detect. Our current understanding is based on 'snapshots' of the whitebait lifecycle at different places and times. Some interesting aspects that are important to the bigger picture are discussed below.
The presence of whitebait shows that adult fish have successfully bred. Whitebait numbers have declined since historical times yet the current trend is impossible to gauge and likely fluctuates between years. The life cycle depends of producing lots of juvenile whitebait, most of which will die. Those making it to adulthood are the breeders for the next generation. Factors that may increase the extinction risk are particularly important. These may act on different life stages and over various time frames. Let’s take a closer look at the three 'extinction events' above:
This is a very good question and some important clues relate to what we know about historical trends. Bear in mind that the actual numbers of fish, eggs, etc are very difficult to detect. Our current understanding is based on 'snapshots' of the whitebait lifecycle at different places and times. Some interesting aspects that are important to the bigger picture are discussed below.
The presence of whitebait shows that adult fish have successfully bred. Whitebait numbers have declined since historical times yet the current trend is impossible to gauge and likely fluctuates between years. The life cycle depends of producing lots of juvenile whitebait, most of which will die. Those making it to adulthood are the breeders for the next generation. Factors that may increase the extinction risk are particularly important. These may act on different life stages and over various time frames. Let’s take a closer look at the three 'extinction events' above:
The whitebait harvest reduces numbers but could it cause #3? The answer is probably not since whitebait migrate throughout the year, and some escape capture during the whitebaiting season. However if the whitebait run became very small then whitebait fishing could have a relatively large effect.
Aside from reducing the number of juveniles through whitebaiting, the link between #3 and #1 largely comes down to habitat. If our rivers offer good habitat then more fish will make it to adulthood and extinction does not occur. Key aspects include adequate food, low pollution and protection from predators.
The spawning habitat is also critical. Eggs are the start of the next generation, but this is a particularly vulnerable life stage. Ensuring the eggs don’t die before hatching will prevent #2 and will increase the stock of larvae that mature into whitebait. Obvious this assists with preventing #3 and therefore also #1. The key activity we can control here is the protection of the spawning sites. Therefore, as well as ensuring good habitat for adult fish we need to be particularly mindful of the 'critical habitat' that is need for successful spawning. Often the locations involved are very specific and occupy relatively small areas. This can make them vulnerable and we need to know where they are.
In summary - extinction should not be seen as a foregone
conclusion. Through attention to the theoretical 'extinction events' it can certainly be prevented. It mainly comes down to the motivation to do it, as well as ensuring that we are following a strategy that actually addresses the main risks.
What does
preventing extinction look like?
Whitebait fishery controls
Reducing the pressure on juvenile fish through a focus on whitebaiting is unfortunately not a panacea because other factors are major influences on the total number of breeders and their eggs. In fact, a preoccupation on controlling whitebaiting might increase extinction risk if it contributed to inaction on other aspects. Because the lifecycle is geared to produce lots of juvenile whitebait, it stands to reason that a sustainable harvest should be possible so long as there is an abundant fish population upstream of the 'back peg' and that these fish are healthy, grow to maturity, and successfully spawn. For these reasons it is imperative that we don’t spiral into a whitebait fishery debate without a holistic view of the other extinction risks.
As with other protection methods, fishery-based strategies need to be monitored for effectiveness and this is no easy task. The most scientifically tractable strategies that involve limiting the fishery are those that can also generate information. This might be achieved by closing several rivers to fishing (to suit the different whitebait species) and monitoring the effects closely, or the use of temporary closures (such as 'no fishing days'). Such ‘rāhui’ might be eventually lifted or a rolling series applied, each river taking its turn. The results could also help judge the value of other fishery restriction proposals in terms of likely effects.
Reducing the pressure on juvenile fish through a focus on whitebaiting is unfortunately not a panacea because other factors are major influences on the total number of breeders and their eggs. In fact, a preoccupation on controlling whitebaiting might increase extinction risk if it contributed to inaction on other aspects. Because the lifecycle is geared to produce lots of juvenile whitebait, it stands to reason that a sustainable harvest should be possible so long as there is an abundant fish population upstream of the 'back peg' and that these fish are healthy, grow to maturity, and successfully spawn. For these reasons it is imperative that we don’t spiral into a whitebait fishery debate without a holistic view of the other extinction risks.
As with other protection methods, fishery-based strategies need to be monitored for effectiveness and this is no easy task. The most scientifically tractable strategies that involve limiting the fishery are those that can also generate information. This might be achieved by closing several rivers to fishing (to suit the different whitebait species) and monitoring the effects closely, or the use of temporary closures (such as 'no fishing days'). Such ‘rāhui’ might be eventually lifted or a rolling series applied, each river taking its turn. The results could also help judge the value of other fishery restriction proposals in terms of likely effects.
Other strategies to increase the number of whitebait
escaping the nets could also be pursued. Again, monitoring is necessary to assess
the actual impacts on adult fish populations upstream. Some of the more obvious options
are:
- outlaw lazy fishing methods such as the ‘Southland Sock’
- reduce net sizes / stream coverage and other details such as traps
impose a catch limit on the amount of whitebait caught- ban the commercial sale of whitebait
- reduce season lengths
