Sometimes observers are charged with taking biological samples, such as otoliths (fish ear bones) and stomach contents. During dissection, other useful information can also be gathered, such as the animal’s sex. Your observer program will provide further instruction for this sampling if it is required.
Otoliths. One of the most important parameters of interest to a fisheries biologist in the study of a sample or catch of fish is the age of the fish taken. Again, observer samples provide this information through the collection of otoliths, or fish ear bones. These bony structures, generally located beneath the brain, show growth rings in a distinct pattern when cut and placed beneath a microscope, which allows the scientist to count the fish's age, in much the same way as a cross section of a tree trunk can reveal a tree's age.
Stomach contents. Understanding a species’ food source allows its location in the greater marine ecosystem’s food web to be identified. When stomach contents are sampled, the tissue is generally preserved in formalin for later analysis in a laboratory.
To be successful as an observer, you must quickly learn to accurately estimate sizeable quantities of fish. There are three ways to estimate a set’s total catch:
- Weigh the entire catch (feasible if the catch is small and there is a brailer scale)
- Use the vessel’s estimate
- Record the brail tally (brail tally × number of brails = total catch weight)
The first option is generally unavailable, and the second option introduces the possibility of bias, given the data’s source. The third option is independent, but requires careful observation. You will find that with experience your ability to accurately estimate catch size becomes better. A brail tally is a record of how full each brail was, which can be added up to give the number of 100 percent full brails. When you multiply that number by the capacity of the brailer (ask the skipper or mate, if you do not know), you have an estimate of the total catch weight. However, the total weight within the brailer can vary with the species and their sizes.