Feature

Where do Whitebait Go?

Understanding whitebait migratory behaviour may help ensure that whitebaiting does not become a thing of the past.

Richard Allibone

Whitebaiting is a major pastime for many New Zealanders. Each spring people fish the lower reaches of rivers hoping to catch some of this New Zealand delicacy. The whitebait runs consist of five species of native fish, all from the Galaxiidae family. The see-through whitebait are juvenile fish returning to freshwater after spending their first winter of life feeding at sea.

The ones that make it past the whitebaiters and other hazards grow to become recognised as inanga, koaro, banded kokopu, shortjawed kokopu and giant kokopu. Inanga, the most common of the whitebait species, is found throughout coastal regions of New Zealand and commonly reaches a size of 7-10 cm. The other species are larger, and less common--adults can be up to 30 cm long and for the giant kokopu over 40 cm long. Individuals of these larger species can reach ages of 10-20 years old.

Over the years much has been said about the decline of the fishery and the possible causes. Deforestation, diversion and pollution of rivers and the introduction of exotic species have contributed to the decline. It has been also been suggested that whitebaiting itself may be a major factor in the decline of the whitebait.

What we don't know is the relative impact of each of these effects. There is continuing lively debate on this issue among scientists, resource managers and fishers.

Fishery managers faced with trying to maintain or restore the whitebait fishery need to know the answers to two difficult questions: how to determine escapement, that is, the number of whitebait that escape being caught? and what level of escapement ensures that enough adults survive to produce next year's whitebait?

But how do you find out the number of fish not caught?

There is still some way to go before these questions can be answered with certainty. What we have been attempting to do is design methods to determine the number of whitebait that get away. We have been testing a mark-recapture method to estimate escapement. In such experiments, a known number of fish are caught, marked and released. If we assume that the marked fish behave the same as other fish, then the number of marked fish not recaptured can be used to determine the percentage of fish escaping.

Simply put, if you release 100 marked fish and 20 are caught again, then 80 or 80% must have escaped. We can then make the assumption that 80% of all fish escape. The difficulty in the whitebait fishery is how do you mark a large number of whitebait without harming them? They are too small to handle and tag without harming them. In our experiments, we used a dye to colour the whitebait red or orange.

We have been testing this mark recapture method in two North Island rivers, the Mokau and Awakino Rivers. The rivers are popular recreational whitebait fisheries with 40-70 whitebaiters in the Awakino and approximately 250 on the Mokau.

For our mark recapture experiments, we captured some whitebait at the river mouths and also obtained live whitebait from some of the whitebaiters. These fish were handled carefully to prevent injuries and kept in fish traps suspended in the river until required. Fish were usually either released within an hour of being captured or kept overnight to release the next morning.

Whitebait lifecycle

Red Fish, Brown Fish

Before the fish were released, they were stained by immersing them in stain solutions of either Bismark brown R (producing orange-coloured fish) or Neutral Red (producing red-coloured fish).

The staining process takes approximately 10 minutes and the fish can be released as soon as they have been coloured. The stained fish were weighed and a sub-sample weighed and counted to allow us to estimate of the total number of fish released.

Releases of stained whitebait in the Awakino Rivers took place at different sites and tidal stages; the two releases made in the Mokau River were at same point and time in the tidal cycle. After each release we spent the next three days visiting whitebaiters on the rivers to record recaptures. The percentage of fish escaping the whitebaiters was calculated from the number of stained fish released and not later recaptured. Movement rates for the recaptured whitebait were also determined when whitebaiters provided the time of recapture. Distances travelled were estimated by measuring the river distance between the release and recapture sites.

The migration rate of whitebait was not constant and was greatly influenced by the tide. We found that the recapture sites were very widely distributed and the degree of escapement also varied considerably. This was attributed to three main factors:

• the state of tide at the release site
• the stain used
• the ability of whitebaiters to recognise stained fish

The stain used affected the time fish remained identifiable, while the state of the tide determined the direction of migration and how long the migration took. The position and number of whitebaiters in the immediate direction of fish movement also affected the chance of capture. Since whitebaiters tended to fish the tide, released fish were subjected to varying levels of fishing pressure.

Upstream, Downstream

In most cases, when released, stained fish dispersed quickly. Some orientated to the current and swam against it. This behaviour meant that in the lower part of the river where a strong incoming tidal flow occurred, some released whitebait would head downstream against the current. For example, one of the releases was made close to the Awakino River mouth on an incoming tide and most of the fish were almost immediately collected downstream by scoopers (whitebaiters using hand-held nets rather than nets at whitebait stands). Similarly, at the Mokau River mouth stained whitebait were caught in the surf zone by scoopers some 9 hours after release, even though the fish were released on an incoming tide two kilometres upstream from the river mouth.

We even had reports of the capture of stained whitebait in neighbouring rivers. One was from as far afield as the Marokopa River, some 40 km north of the Awakino River! Other whitebait appeared to head upstream with incoming tides, essentially riding the tide. These fish could move rapidly up the river at over a kilometre an hour.

Observations by whitebaiters of shoals of whitebait swimming downstream during tidal pushes also suggest that whitebait may move up and downstream in the lower parts of rivers. If this represents standard behaviour of migrating whitebait then this exposes the whitebait to much greater fishing pressure than if they migrate up-river constantly.

Migration rate is distinct from swimming speed. Swimming speed is the speed the fish can achieve under the conditions prevailing at the time. The migration rate includes swimming speed but also accounts for fish behaviour and river conditions. As whitebait do not migrate at night, upstream movement halts for the hours of darkness. This leads to reduced migration rates when comparing fish captured on Day 1 of a release with subsequent days. Furthermore, any downstream movement by the fish as tidal flows change will decrease the upstream migration rate. Obviously the faster the migration rate, the less time the fish are available to be captured by whitebaiters.

Using the time from release to recapture, we calculated that on the first day after a release, the rate of upstream migration ranged from 0.06 to 1.42 km/h. The large variation in migration rate was due to the differences in behaviour of fish upon release. Some fish moved upstream rapidly, using incoming tides to push them upstream. Conversely other fish that swam against the tidal current were often recaptured relatively close to the initial release point.

The upstream migration rate calculated for the first two days after a release though was generally much lower, ranging from 0.15 to 0.51 km/h. At the end of Day 2 most, or all, of the released fish that were recaptured had made nett inland movements, and the differences in distance covered by individual fish became less pronounced.

The whitebaiters recaptured between 1.3% to 44.6% of the dyed whitebait in the Awakino River, with a mean for the eight releases of about 19%. For the two releases in the Mokau River, the recaptures rates were 5% and 12%. The fish stained with the Neutral Red dye were recaptured less frequently (1.3 to 20.6 % recaptures) than the Bismark brown R dyed fish (7.4 to 44.6% recaptures). This was most likely due to the Neutral Red dye fading quicker that the Bismark brown R dye.

These recorded recapture rates also represent the minimum capture rates of whitebait. Factors that increased the real catch rate included:

• deaths of stained fish
• unrecognised or unreported captures of stained fish
• the capture of stained fish once the dye had faded.

Therefore whitebaiters can possibly catch over 50% of the fish in the migrating upstream on some days.

Interestingly, catch rates were not always greatest during the initial fishing period after releases. Peak capture rates were often on the second fishing tide after releases. This often coincided with increased fishing pressure in the mid-tidal reaches of the rivers and channel forms forcing whitebait to swim closer to banks therefore being more easily caught. Also of interest was that recaptures of stained whitebait occurred on both sides of the rivers, not just the bank from which releases were made. This indicated that whitebait regularly crossed the river channel.

There is one final step to determine the number of whitebait not being caught and we require one further bit of information. This is to find out not just how many stained whitebait are caught, but also the total number of whitebait caught. Then we will know the percentage of stained whitebait escaping and the total number of whitebait caught and we can calculate the total number of whitebait escaping the whitebaiters. To do this we need to know what many whitebaiters regard as secret, just how much they catch and how good is their fishing spot.

Richard Allibone is a researcher with NIWA in Hamilton.