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Life History of Reef Fishes

We aim to discover key aspects of life history for the target species of this project: size at maturity and seasonality of spawning. These two life history characteristics are essential to understanding species biology and among the most actionable with respect to fishery management, since they can be used to determine size limits and seasons.

How can you help? Please contribute gonad samples of your munu and moano kali to us! We will take measurements and extract the gonads, and then you will have your fish back to take home for dinner. Me, I usually go for steamed with watercress and boiling peanut oil !

We are also targeting 5 species of nenue which can be difficult to tell apart. There are a few indicators that can help us to distinguish between the different species, such as the shape of the fins, dorsal soft ray count, coloration, and length of the pelvic fin. The shape of the fins and the coloration are quick and easy metrics but are usually not decisive or objective enough to make a positive ID.

Kyphosus cinerascens has an anal fin that creates a right angle which sets it apart from the other species of nenue.
On the other hand, it’s difficult to use the angle of the fins on this fish because it’s not clear how flared the fins are. The coloration also isn’t stark enough to make a positive identification. This could potentially be Kyphosus sectactrix, Kyphosus hawaiiensis, or Kyphosus elegans!

Fortunately, we can also count the number of soft dorsal rays for more objective identification.

Soft rays refer to the bones in the fin that don’t come out past the edge of the soft part of the fin. Having the fin backlit makes the rays easier to count. This particular fish has 12 soft dorsal rays, eliminating the possibility that it could be Kyphosus vaigiensis.

Size at maturity is determined by sampling individuals across a size range and determining which ones are sexually mature. The keiki fish have tiny gonads that weigh almost nothing in comparison to their body weight. Once they become sexually mature, the gonads become much larger. The ratio of gonad weight to body weight is called “gonadosomatic index”, or GSI. If we draw a graph of GSI vs. length, we can see that it suddenly goes up at the length of maturity.

Preliminary results suggest that maturity may occur at around 250 mm fork length

So how do we do this? We carefully dissect out the gonads, and the weigh them on a scale that can detect one thousandth of a gram … that is a very small amount of weight, so a slight breath of wind will make the scale dance all over.

Ovaries inside a moano kali (ovaries are the female gonads)
Here the gonads are placed on the head of the fish to show their size

We also take some of the eggs and look at them under a microscope. This allows us to tell if the fish is getting ready for spawning. In the runup to spawning, the eggs change from tiny clear spheres into much larger dark colored spheres full of granules.

The eggs of a moano kali. PG means ‘primary growth’, indicating the starting stage, i.e., not ready for spawning. VTG means ‘vitellogenic’, which indicates that the fish is getting these eggs ready for spawning.

Once the eggs have developed through vitellogenesis they become large and transparent with a single yolk globule. Hydrated eggs indicate that the fish is currently spawning.

Hydrated eggs from a Munu caught in Waimanalo in January

We record the time of year when each fish was caught so we can determine the seasonality of spawning. Plotting the month against the GSI helps us visualize what time of year the relative gonad size is largest.

Preliminary results indicate a spring spawning season

All photographs by Kevin Weng unless otherwise noted, and all copyrighted. Please contact me for use!