Fishing News index>August 2007

Otolith Fingerprinting
By Ricky Olley, Masters Student
Otago University

Recent research has focused on the use of natural tags or trace element fingerprints to track the movements of trout. One of the most useful parts of a fish for examining such natural fingerprints is the otolith, or ear bone found within the inner ear of all fish. Otoliths are accretions of calcium carbonate that grow continuously over the life of the fish. Concentric layers of new material are deposited every day, and although consisting almost entirely of calcium carbonate, other elements may be trapped within the crystal lattice. As the fish moves between different chemical environments within a river system, the composition of layers within the otolith also change. Potentially, fish hold a permanent record of the different chemical environments in which they have lived locked within their otoliths.

Unlocking the information contained within otoliths can be done using a technique known as laser ablation. A narrow powerful laser beam is fired at the otolith, evaporating a tiny amount of material from the otolith surface. The gas evaporated from the otolith can then be analysed in detail. By cutting an otolith in half we can expose a surface that runs from the core (formed when the fish hatched) to the edge (deposited just before the fish died). Hence, by running the laser beam from the core to the edge, we can determine the chemical environments in which the fish lived over its entire life.

The Motueka Study
This technique was applied to the Motueka River as a joint project between Nelson Marlborough Fish and Game, The University of Otago and The Cawthron Institute. The aims of this study were two fold, firstly, to create an element map of the catchment by examining the otoliths of juvenile fish from a range of tributaries, and secondly, to use the map to trace adult fish back to specific tributaries to gain an understanding of recruitment processes occurring throughout the catchment.

To create the fingerprint map eight tributaries representing most of the major sub-catchments of the Motueka were sampled. Young-of-year fish were collected from the Upper Motueka at the bottom of the gorge, Blue Glen, Motupiko, Rainy, Dart, Upper Wanagpeka, Baton and Graham Rivers. As it is unlikely that these young fish would have moved since their emergence from the redd, the element fingerprint in their otolths provided a reference for the trace element signature of the stream where they were caught.

To define the fingerprint map, a statistical test known as Discriminat Function Analysis was used. This process takes each juvenile fish and tests how well it matches the tributary it was caught in based on the differing concentrations of trace elements found in its otolith. The analysis gives a classification success for each river suggesting how many of the fish taken from it grouped to it in the analyses. We plotted the output of the analysis onto a graph to gain an idea of how the signatures of the tributaries compare to each other. This can be seen in the following figure; the scales on each axis represent variation in the levels of multiple trace elements and the ellipses represent the spread of the data around the mean for each tributary thus giving a summarised view of that set of points.

Overall the analysis of juvenile otoliths successfully classified 94.7% of the juvenile fish to the tributary they were caught. The only two tributaries that couldn’t be classified at 100% were the Baton at 93% and the Upper Wanagapeka at 60%. Such a good classification suggests that not only are the element levels in the otoliths unique to each tributary, but also that the differences between the element signatures of the tributaries is good enough to individually define almost all eight tributaries.

Fourty eight adult fish were then sampled from the main stem of the Motueka by a team of anglers. From these fish we examined the chemical composition of the otolith material laid down when the fish was a juvenile growing in its rearing stream. This data was added into the analyses un-grouped having an un-know origin. The analyses then attempted to fit these fish into one of the groups defined by the juvenile otoliths and gave which tributary each adult fish was most likely to have come from, and a probability defining how well it matched it. Only those fish that matched a tributary with a probability of 95% were considered to have come from that tributary.

Sixty percent of the adult fish caught could be traced back to one of the eight tributaries sampled. Specifically; 11 traced to the Baton, eight to Blue Glen, seven to the Dart, two to the Rainy, one from the Motupiko, one from the Upper Motueka, and none from either the Graham, or the Upper Wangapeka. This can be seen in the figure below. The red area of the pie charts represents the number of adult fish tracing back to each of the tributaries.


Conclusions
This result goes some way to describe which parts of the Motueka Catchment are supplying fish to the main stem of the river. It also poses some interesting questions. It would appear the Baton supplies a large proportion of the recruits, but the spread in the data within this group suggests certain parts of the Baton may be more important than others. Blue Glen stream also comes out as important, a slightly surprising result for its size, and suggests that the smaller tributaries within the catchment may well be important in their own right. The relatively high contribution from the Dart is also a surprise. Conversely such low numbers from the Rainy and the Motupiko, rivers that are well know for their spawning habitat is a strange result, and more research such as that of Jacob Lucas exploring juvenile movement in the Rainy may cast some light on this puzzle. The lack of contribution by the Upper Motueka, and the Graham are perhaps not unusual given the lack of suitable spawning habitat within these rivers.

Furthermore, it seems recruitment into the main stem can occur over large distances confirming the notion that brown trout can be a highly mobile species. For example, adult fish that were caught near the mouth of the Motueka River could be traced back to tributaries in the upper reaches such as Blue Glen, a migration distance upwards of 80km.

Although it is tempting to take the numbers of fish from each tributary outputted by our analysis at face value we have to approach them with a certain level of caution. There are many other tributaries and sections of the main stem that were not examined in this study and although this could provide an answer to the origin of the 40% of the adults that couldn’t be traced, the addition of further tributaries to the analyses could also alter the pattern through cross-over in element signatures between sites, a problem inherent to this method.

However, this is one of the first times this method of tracking trout movement has been applied to a New Zealand river and when compared to studies from other parts of the globe it has performed remarkably well. It seems likely that with further examination and refinement of the method we should be able to confirm or disprove the patterns we are beginning to establish, and answer a number of the questions that remain. It is exciting to think of the many other rivers that could benefit from such work, and to the possibility of getting that little bit closer to understanding the movement mysteries of the wily old trout.

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