Just like other fish species, larval and juvenile yellowtail kingfish grow very rapidly. After hatching, the larval fish eat small planktonic animals. During the first few weeks of feeding there is large variation in growth rates between individuals. It is difficult to study small larval fish in the wide expanse of the ocean, but you can see just how much size variation can occur when larvae are raised in tanks for aquaculture. Below is an example of the size variation seen in 20 day post hatch (DPH) juvenile yellowtail kingfish from a rearing tank at the NIWA Bream Bay Aquculture Park.

Aggression and cannibalism can be intense in juvenile fish. In the above picture you can see a large individual with a smaller one in its mouth. The beginning of juvenile aggression is observed as larger individuals assuming a J-shaped attack posture while tracking smaller subordinates. This behaviour starts at around 12 DPH.

At around 18 DPH some of the large individuals start chasing and nipping other juveniles.

From 30 DPH you can observe large fish ingesting smaller fish. This can result in both the predator and prey dying, as the larger juveniles are not yet able to properly swallow their prey and they have a poor ability to regurgitate.

Not all of the large individuals are equally aggressive, some are clearly more aggressive than others. If these juvenile cohorts are not size graded the cannibalism can wipe out most of the cohort. I undertook a trial to investigate just how effective size grading was in reducing aggression and improving the growth of the smaller individuals, which are most often the recipients of aggression.

Juveniles from a single cohort were graded into three sizes at 28 DPH: small, medium and large. The large fish were easily recognisable as exceptionally large individuals, and the small individuals were very small and often yellowish in colour. The medium size grade where those that were in between the large and small grade juveniles.

The cohort that was graded had the following size distribution:

The juveniles were grown for 12 days in size graded and ungraded (control) treatments. Growth rate, aggressive behaviour and RNA:DNA ratio were monitored during the trial. There was a statistically significant increase in average weight for groups except the small grade of juveniles. The smallest juvenile recorded in the control group at the end of the trial was bigger than most of the juveniles in the small grade.

Aggressive behaviour was only seen in the control groups. The large grade juveniles did not show aggression, indicating that size variation is a key factor in promoting aggressive behaviour. The medium grade did not display any aggressive behaviour, despite having a similar average weight and size variation as the control groups at the end of the trial. The conclusion is that aggression is only carried out by some fast growing individuals when there is size variation.

Mortality was high in the medium and small grade, even though no aggressive behaviour was observed in these groups. The control group had a similar survival profile to the medium and small grade, which was probably related to the fact that the ungraded population was mostly composed of small and medium individuals. The large juveniles had a much higher survival rate than all of these groups.

The RNA:DNA ratio was used to measure the growth rate of individuals before and after grading. While the amount of DNA is fixed per cell, the amount of RNA varies depending how much protein is being produced (in a process called transcription). Juvenile fish grow very fast and have a high proportion of muscle in their body, and muscle is made of protein. If one measures a high mass of RNA compared to DNA, it means a lot of protein is being produced and the fish is growing fast (e.g. RNA=3, DNA=1). Conversely, if growth is low, the protein production rate will be lower (e.g. RNA=1, DNA=1). The RNA:DNA ratio can give you a snapshot of the relative growth rate at one time point, something that can't be derived from a single weight measurement.

Below is a graph of the RNA:DNA ratio of the different size grades before and after the grading trial. The small grade decreased in RNA:DNA ratio to a value near 1, indicative of starvation. There was a clear correlation between RNA:DNA ratio and weight for the other size grades. This goes against what one may have predicted, as the correlation implies that faster growing fish (i.e. the large grade) have the same RNA:DNA ratio as slower growing fish for any given weight. If RNA:DNA ratio was a useful reflection of instantaneous growth rate, it would be expected that the faster growing fish would have a higher RNA:DNA ratio than fish with lower growth rates for any given weight. One explanation for the results is that the while the amount of RNA present may be similar for two fish of different growth rates at any given weight (i.e. the same RNA:DNA ratio), the rate of RNA turnover may be higher in the faster growing fish. Alternatively, the RNA:DNA ratio may be constrained by upstream processes that better reflect growth rate.

In summary:

Aggression was carried out by a small number of fast growing individuals
Slow growing individuals did not survive when aggressive individuals are present
Most slow growing individuals were on a degenerate developmental pathway irrespective of the aggessive environment
RNA:DNA ratio was not a useful proxy for instantaneous growth rate

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This research was funded by the NZ Foundation for Research Science and Technology, and I was supported by a NZ Tertiary Education Commission Bright Futures Doctoral Scholarship.

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This research has been published and is available as follows:

Moran, D. (2007). Size heterogeneity, growth potential and aggression in juvenile yellowtail kingfish (Seriola lalandi Valenciennes). Aquaculture Research 38: 1254-1264.

doi:10.1111/j.1365-2109.2007.01769.x ------- PDF of paper