NUMBER CRUNCHING

A spotlight on scientists who are turning ‘data’ into useful ‘information’.

One of the difficulties of studying the diet of marine mammals is the inability to see them in their own environment. What are they doing while underwater? What are they eating?

For marine mammals that spend part of their time on land (seals, sea lions, walruses, polar bears), the time they spend on land can be used to learn more about their eating habits. This can be done by collecting and examining their fecal samples, also known as scat samples.

Mandy Wong is determining the diet of Hawaiian monk seals using scat samples. Hard parts from the monk seal prey, such as fish bones or cephalopod beaks can be recovered from sieves after washing away the fecal material. While not the most glamorous or cleanest of jobs, analyzing the hard parts retrieved from the fecal samples can be like piecing together a puzzle.

Fish are placed into a “bug box” to remove the flesh from the bones. These bones are used as a reference collection.

Recovered hard parts are sorted and compared to the reference collection to identify the type of prey consumed by the monk seals. Digestion will sometimes erode away portions of the bone making identification more difficult. The number of samples a prey type is found in is referred to as the “Frequency of Occurrence.” The hard parts can also be counted to determine the number of individuals consumed for fish and cephalopods.

Example of prey hard parts recovered in scat samples

From 576 monk seal fecal samples collected between 1995 and 2007, Mandy recovered hard parts from more than 9 000 prey individuals. That’s over 70 000 hard parts! These hard parts came from at least 36 families of fish and 3 families of cephalopods. The most common fish families in the diet were Labridae (Wrasses), Congridae (Conger eels), Balistidae (Triggerfish), Scaridae (Parrotfish), Holocentridae (Squirrelfish and Soldierfish), Muraenidae (Moray eels), and Ophichthidae (Snake eels). These families all have members which are commonly seen when snorkelling or scuba diving over shallow coral reefs in Hawaii and other areas of the tropical Pacific.

Figure 1. The mean number of individuals consumed per sample and frequency of occurrence for prey occurring in at least 5% of samples.

This might lead you to believe that Hawaiian monk seals search for food mainly on shallow coral reefs. However, very few of the prey are found exclusively in coral reefs. Instead, it appears as if sediment and sandy bottoms are important foraging areas for monk seals. The data also suggest that monk seals prefer to search for food along the bottom of the ocean floor rather than in the water column and that they hunt equally during the day and the night. Another interesting finding is that the prey of monk seals are also mainly carnivorous.

Figure 2: The habitat, activity, evasion, and foraging type for prey occurring in at least 5% of samples.

So even though it is difficult to see what monk seals are doing while underwater, evidence in their fecal samples can give insight into their diet and foraging habits.

Mandy Wong is a Masters student at U.B.C.