A storyteller called otolith and fisheries biology

By Natasha Travenisk Hoff

English edit by Carla Elliff

*post originally published in Portuguese on October 8, 2020

In addition to being time tellers, how can otoliths help in fisheries biology?

Illustration by Joana Ho.

In the world of bony fish, otoliths can be considered true storytellers: these calcified structures, present in the form of three pairs (sagitta, lapillus and asteriscus) in the fish's head and responsible for balance and sound perception, they grow in layers of carbonate and protein with daily deposition (like an onion, you know?). These structures allow us, scientists, to make inferences about their eating habits, what type of environment they were born in, whether they migrate, among many other possibilities. The most common and well-known use is in fish growth studies. In the post “How to know the age of a fish and other things...”, Chat with Neptune editor, Claudia Namiki, talks about what otoliths are and how much information these small structures carry.

Side view of the inner ear of a teleost fish, including the position of the three pairs of otoliths (sagitta, asteriscos and lapillus), and their location in the head.

(Illustration by Natasha Travenisk Hoff, license CC-AS-BY 4.0).

Generally, those who start working with otoliths begin their studies by analyzing the growth of their rings during their undergraduate or master's degrees, moving on to other types of analysis later... I only started my work with otoliths during my doctorate, in 2015, analyzing their shape and chemical composition applied to fisheries oceanography! Everything was effectively new to me, which represented a great personal and professional challenge! And the most interesting thing is that these little structures always bring me new curiosities, applications, questions... and that makes me want to know more about them every day!

What I bring to you today was a chapter from my doctorate, recently published in the Journal of Applied Ichthyology, in which we sought to evaluate, through the shape of the otolith, the population structure of the bigtooth corvina (Isopisthus parvipinnis, a cousin of the weakfish that is widely caught even though it is not a target species or having much commercial value), from the northern coast of the state of São Paulo to Santa Catarina, divided into five sub-regions, at two very different times: in 1975 and 2018/2019. In other words, we wanted to know if the species has different population units (or fishery stocks) and if these varied over these 43 years, speculating about the causes of these variations.

But “how important is this?”, you ask. When we talk about fisheries management, knowledge about different stocks helps us organize fishing and assess the possibility of recovering a species in the face of very intense exploitation (overexploitation), whether or not this species is the targeted species, or changes to the natural environment or due to human action. For example, if the population of a species is restricted to the Santos region, it would be much more vulnerable than if it were distributed across the coast of São Paulo.

This is the bigtooth corvina (Isopisthus parvipinnis, Sciaenidae) and a map containing the locations where the individuals I analyzed were captured in 1975 and 2018/2019 in the coastal region of the states of São Paulo, Paraná and Santa Catarina. Photo and map: Natasha Travenisk Hoff, license CC-AS-BY 4.0.

The shape of the otoliths of each species is unique and, therefore, very useful in studies on the feeding habits of organisms that feed on fish, but the factors that determine this shape have not yet been completely understood. What is known is that there is a strong genetic component, which determines the general shape, but that there is also variability related to sex, age, diet, environmental conditions, and other factors such as water temperature, depth, type of substrate (mud, sand, gravel, etc.). Due to all these influences, population units were found to be recognizable and distinguishable through the variations in otoliths of the same species!

Example of different shapes of otoliths. Source: COSS – Brasil (http://usp.br/cossbrasil/glossario.php), license CC-BY 3.0.

And these variations were observed by us, not only spatially, but also between the two periods studied. Look what we found:

1. In 1975, the bigtooth corvina formed a single population unit. Even living in environments with different oceanographic characteristics, whether under the influence of large estuaries in the south of São Paulo and Paraná (Cananéia and Paranaguá, respectively), or due to the resurgence phenomenon, which affects the north of São Paulo and the region of Santa Catarina.

2. In 2018/2019, a new condition was observed, with the formation of four population units: one in the north of SP, another in the center of SP, a third between the south of São Paulo and Paraná, and a fourth in Santa Catarina.

And what could be the causes for such changes? We attribute this differentiation between locations and years to the different ecosystems and oceanographic processes found in the study area, and to changes in the coastline, which have altered the contribution of river waters to the coastal environment in the last 40-50 years, as we can see in the figure below.

Evolutionary scheme of the mouth of the Guaraú River (near the collection site in Peruíbe, SP) in the years 1966, 1973, 1981 (Tessler et al., 2006, available at www.mma.gov.br/publicacoes-mma ) and 2020 (Google Maps). 

Fish, like all living beings, are capable of adapting to natural environmental changes or those induced by human action through modifications in their physiology and behavior, which are not necessarily related to genetic changes, but which directly affect reproduction, morphology or survival. For this reason, it is important to emphasize that our results are not necessarily evidence of genetic differentiation.

The use of otoliths to evaluate fish stocks in Brazil is still in its infancy and no other Brazilian species had been approached in the way we did, comparing samples so old with current ones. As this was the first analysis with this species, it is also important that other methods are tested to support and/or challenge the results obtained. Spoiler alert: the analysis of the chemical composition of these same otoliths presented very similar results, but with more confidence in the distinction of population units and also distinguished Santa Catarina from the other locations studied in 1975.

In itself, this study raises many other questions, such as:

1. Did this same process happen with other species? 

2. Can we see any impact on fishing, given that stocks can behave in different ways in the face of fishing pressure?

3. Could the decrease in the number of bigtooth corvina captured recently be a reflection of this process? It could be, since the species occurs as an accidental capture and little attention is directed to it and so many other species in the same situation.

4. Knowing that changes in the coastal zone due to human action can directly and indirectly impact fish populations, what can we, as a society, do in the face of these changes? Be a critical citizen: seek to know the origin of the fish you consume (why does meat need seals to be sold while fish does not?); whether they are threatened species; support local fisheries; if the sewage of the coastal city you like to visit is treated or the condominium/building/house you are staying in by the sea has a license...

Anyway, it is clear that there are natural variations occurring in the ocean that could lead to this distinction between populations, but we have to be aware of the influence that our society exerts (and more so every day!) on different ecosystems, and take responsibility for changes that improve these relationships between human beings and the environment, especially the marine environment!

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References or reading suggestion

Coleção de Otólitos de Peixes Teleósteos da Região Sudeste-Sul do Brasil (COSS – Brasil). Site: www.usp.br/cossbrasil/

Hoff, N.T., Dias, J.F., Zani-Teixeira, M.L., Correia, A.T. 2020. Spatio-temporal evaluation of the population structure of the bigtooth corvina Isopisthus parvipinnis from Southwest Atlantic Ocean using otolith shape signatures. Journal of Applied Ichthyology, 36: 439-450. doi.org/10.1111/jai.14044

Tessler, M.G., Goya, S.C., Yoshikawa, P.S., Hurtado, S.N. 2006. São Paulo,in Muehe, D. (org.), Erosão e progradação no litoral brasileiro. Brasília: MMA. pp. 297-346. www.mma.gov.br/publicacoes-mma 

Vignon, M. 2012. Ontogenetic trajectories of otolith shape during shift in habitat use: Interaction between otolith growth and environment. Journal of Experimental Marine Biology and Ecology, 420-421(2012):26-32. doi:10.1016/j.jembe.2012.03.021

Volpedo, A.V., Vaz-dos-Santos, A.M. 2015. Métodos de estudos com otólitos: princípios e aplicações. Ciudad Autónoma de Buenos Aires: PIESCE - SPU.

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About the author

Passionate about the sea, music (playing or dancing, I've even given ballroom dancing classes!), and family. I am an oceanographer with a degree from the University of São Paulo, and a master's degree in Science (Oceanography, area of ​​concentration in Biological Oceanography) from the Oceanographic Institute of the University of São Paulo. Currently, I am in the final phase of my doctorate at the same institute, with inorganic chemistry, fish ecology, otoliths, and biotic integrity of fish communities being my main lines of research. 

Contato: tashahoff@gmail.com

#FisheryStocks #Otoliths #IsopisthusParvipinnis #MarineScience #Oceanography #FisheryBiology #MarineBiology #Fisheries #NatashaHoffChat