Original Paper: Moore, B.R., Lestari, P., Cutmore, S.C., Proctor, C. and Lester, R.J., 2019. Movement of juvenile tuna deduced from parasite data. ICES Journal of Marine Science, 76(6), pp.1678-1689. https://doi.org/10.1093/icesjms/fsz022
When you think of parasites, your first thoughts probably aren’t “helpful” or “useful.” However, parasites aren’t just something we try to get rid of; they can be studied and used in all kinds of applications, including conservation.
Parasites as Indicators
Parasites by definition form close relationships with their plant or animal hosts. Thus, by analyzing parasites, scientists can learn about their host’s movement, surrounding environmental conditions, and time spent in various locations. For this reason, parasites are currently being used to address a number of conservation problems, including overexploitation, habitat loss and fragmentation, invasive species, and climate change.
Parasites have become an important conservation tool to measure the health of fisheries. For example, many studies have already successfully used parasites to determine the movement and resulting stock structure of fish and invertebrates in a variety of marine environments. The idea behind using parasites as biological tags is that fish and invertebrates which reside in the same environments or share common histories should be hosts to similar parasites. Therefore, by identifying the parasite species and their residence times in certain fish and invertebrates, scientists can determine the movement history of those fish and invertebrate species.
The Secret Lives of Tuna
Tuna species are important sources of revenue and food to people around the world. In particular, bigeye tuna (Thunnus obesus) and yellowfin tuna (Thunnus albacares) contribute significantly to the livelihoods of those living in and around the Indian and Pacific Oceans. These two species are some of the most harvested in the area, with the catch of both species together totaling to about 1.6 million metric tonnes across both oceans in 2016. Therefore, proper management of fishing stocks in these oceans is especially important for the persistence of these tuna species. Although tuna are such important food sources for many regions, scientists do not fully understand their movement across their geographical distribution. This complicates management and conservation for fisheries. Recent studies have shown that these tuna species may form several distinct populations and that their movement might be restricted to certain geographical regions. Using parasites as biological tags, scientists hope to learn more about movement of these different populations, and these movement patterns could inform future tuna management plans.
A group of scientists led by Dr. Bradley Moore were interested specifically in using parasites to determine tuna movement in relation to Indonesian fisheries because of the importance of tuna to Indonesia’s economy. Indonesia is developing harvest strategies for its 11 distinct fisheries management areas (FMAs), so results from this study could inform the creation of these strategies. The scientists collected information on parasites that were found in juvenile bigeye and yellowfin tuna in marine areas of Indonesia’s Exclusive Economic Zone (EEZ) during two consecutive years (2013 and 2014). They collected fish from six FMAs and two outlier areas (the Maldives and the Solomon Islands) which were chosen to serve as contrasts to the results of the FMAs (Look at Figure 1 in the paper). They bought fish of the same age and size from fish markets and distribution companies at the ports at each site, and they used information from those sources to determine where the fish were caught. They then identified the number and types of parasites in each fish to see if there were any similarities in the community of parasites in fish caught in similar areas. By comparing where fish were caught and their types of parasites, scientists hope to learn more about the movement patterns of these fish.
Parasites Reveal Limited Movement Patterns
The researchers identified eleven parasite species found in the two tuna species, with seven parasite types ultimately used in the analyses. They found consistent differences between geographical areas and parasite abundance, though the differences varied by parasite type and tuna species. Bigeye and yellowfin tuna had somewhat different abundances of certain parasite species among FMAs, but the overall patterns the parasites displayed were very similar. Patterns from these results indicated that the fish from outlier sites (the Solomon Islands and the Maldives) had different movement histories from the neighboring Indonesian fish. For both tuna species, the results also suggested there were specific movement patterns within the Indonesian FMAs; mainly they demonstrated there was limited movement of the fish from the Indonesian archipelago into the eastern Indian Ocean (Look at Figure 1 in the paper). There was some mixing of fish from different environments found at a few FMAs along regional borders, but these fish ultimately were faithful to their group and so otherwise moved within the same environment as other fish with similar histories. In addition, the absence of key parasites in specific fish ended up being more influential than the community composition of parasites when the movement patterns were modeled.
A Fishery in Trouble?
An important dimension of the health of fisheries is the number of fish (or stock) found in different areas. Currently, most assessments of fish stocks are based on the idea that fish of the same species are part of the same group or population. Scientists believe that when one location within a larger geographical area is overharvested, then fish from the general population (mainly from neighboring areas) would replenish the stock of the overharvested location. However, the movement patterns inferred from these parasite data suggest fish found in certain regions are not regularly traveling to outlier areas and mixing with other groups of fish. Tuna are known to be able to travel over 800 miles per month, and so if these locations are part of the same larger population, neighboring fish populations could easily replenish overharvested areas. The lack of movement from other regions could be due to local oceanographic conditions, habitat preferences, and availability of prey resources at each site where the fish were collected. This information could help determine the rate at which specific stocks of fish would be replenished once harvested, as fish with the same movement patterns (not necessarily those in neighboring areas) are most likely to replenish those stocks. This has implications for Indonesia’s harvest strategies and policies regarding fish stocks in that area.
The Future of Fish Parasites
Though often unwanted visitors, parasites in fish and other organisms can be extremely helpful when used as biological tools to examine their host’s biology. For fish, this technique of using parasites to determine movement patterns is becoming an important way to gather more information about fish stocks in particular, which is crucial for communities all over the world. By combining the data from fish parasites with other types of data like otolith (fish ear bone) chemistry, tagging data, and genetic approaches, governments and agencies can develop appropriate management strategies for the tuna species in this paper, as well as for other fish. If you want to know more about how parasites are being used in a range of biological and conservation applications to further scientific knowledge, check out this review paper on the use of parasites as conservation tools.