One fish, two fish, red fish, blue fish!
Following along the theme of the decor in the large lounge, today I want to talk about fish. Specifically larval fish! We have a group on board, led by Dr. Iain Suthers looking at larval fish. Using a 1 meter by 1 meter net, towed horizontally through the water column, they can compare the catch at different depths. Called the “E-Z” net (or “E Zed” locally), this net has 5 compartments that can be triggered individually. Setting the depth of the tow is important, as depth is a critical parameter defining habitat for these larval fish.
Larval sardines and copepods (type of zooplankton, pinkish small between sardines; photo courtesy of Iain Suthers)
The group is largely interested in the larval stage of fish with economic significance including sardines, mackeral, and yellow-tail scad. These species spawn in the spring, with the larval stage lasting between 2 and 4 weeks. During this larval stage, the ear bones (called “otoliths”) are growing in daily increments- visible under a microscope you can count the daily growth lines to determine age. Like counting tree rings, but in days instead of years. Larval fish are extremely useful in classifying habitats. While non-conservative (they change in a given habitat, only there in short periods of time), larval fish can be as telling about a habitat as temperature and salinity can be for water mass identification.
Two mackeral larvae (bottom left and bottom right) and one flounder larvae (top left, photo courtesy of Iain Suthers)
The big question for larval fish is in terms of their survival- mortality rates are estimated as high as 10% a day–thats a lot of dead little fishies! A primary objective to answering this question is to understand the relationship between growth and mortality. More specifically, where are the nursery grounds and how does survival vary by nursery. Traditionally, the regions near shore (sea grass beds, estuaries) have been the focus of nursery studies–but larval fish in these regions experience large predation pressures, and thus high mortality. However, until recently, being washed out to the Tasman Sea into the more open-ocean waters has been viewed as a death sentence for larval fish. In reality, it may be just the opposite. Observations have shown much lower mortality rates in the open waters, and once the larval fish get big (and strong) enough then can swim back to the shallower regions. This hypothesis can be tested using those tiny otoliths. Its pretty straight forward to look at growth rate, whether thats in millimeters (length) or milligrams (weight).
Larval gurnard caught this cruise (left, photo Iain Suthers), adult gurnard for comparison (right, photo from Chris Stanley)
Getting at mortality is a bit trickier, and there are a lot fewer scientists who try. But arguably, mortality is the more important factor. High mortality means a greater selection pressure- if fewer survive, they will be the “Olympians” of the fish world. That is, only the very fastest very strongest will survive when survival rates are low. The quicker a larval fish can grow, the quicker they get out of the “most at risk” size window. These are the ones likely to survive high selection pressures. This can skew growth rate measurements as the results are kind of counterintuitive – a small catch will have the highest growth rates (because the slower ones have been eaten) and a big catch will be average (more slow growers survived).
Sardine larvae(top), two larval mackerals, and a copepod (small pinkish critter in between the two mackerals; type of zooplankton)
If the larval fish washed out to sea are surviving, and surviving in large numbers, then the open ocean is an important player in the survival of commercial fisheries.