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The little fish hidden in tide pools

By Marina Brenha-Nunes

This story begins in the Araçá Bay, which is a tidal flat located in São Sebastião, on the northern coast of São Paulo. This flat remains almost entirely uncovered during low tide, but at high tide, it is home to large numbers of fish and rays, which enter the plain to feed and reproduce. The tidal flat is next to the Port of São Sebastião, which expands into the bay area. Scientists from various fields are studying this area in order to serve as a model for other sites that need to develop a management and local development plan.

When working with coastal fish, it is very common to collect them during high tide, mainly for two reasons: the first is that at high tide there are more fish close to shore; the second is that many collecting techniques work to capture as many fish as possible in a given area. These techniques include trawling, using a motor boat, and the “picaré” net (beach seining) (see more about fish and beaches here).

During an afternoon of intense work collecting fish at high tide in the Araçá Bay, Professor Carmen Rossi-Wongtschowski (my supervisor), who was observing and supervising everything, noticed that when the tide was going down, some water bodies remained isolated, disconnected from the sea, by the sandy/muddy beach. She also observed that there were many fish living in these pools, and that the samples taken at high tide were not capturing these animals, which also belonged to the study site of Araçá.

We talked and talked... we decided that my master's project would be to study the fish that inhabited these bodies of water in the Araçá Bay.

Actually, there are already studies with fish that inhabit these bodies of water, which are called tide pools. However, we realized that in Araçá there were two types of tide pools, sandy and rocky. Observing this difference, we began to suspect that this fact could bring variation in the species and abundance of fish that lived in the two types of pools. So that is what we did, we identified which and how many species were present in each type of pool.

Photograph of a coastal landscape with mountains in the background on a sunny day. In the foreground there is a group of 5 people observing a sandy pool surrounded by rocks

Sandy pool - Example of a sandy pool used for fish collection in the Araçá Bay.

(Photograph by Marina Brenha-Nunes, CC BY-SA)

Photograph showing a rocky pool with a 1-m scale (total width of the pool is approximately 5 m)

Rocky pool - Example of a rocky pool used for fish collection in Araçá Bay.

(Photograph by Marina Brenha-Nunes, CC BY-SA)

After reviewing the literature on fish in tide pools, we found that most studies were on rocky pools, with no studies dedicated to sandy pools. This fact caught our attention - why are there no studies on other kinds of pools?

We clearly identified two types of pools in the same place, however there were no references that "enlightened" us as to how to collect fish in sandy pools. So, off we went! We developed a protocol for collecting fish from sandy pools that should meet some basic requirements: (1) be reproducible for sandy pools found anywhere on the planet; (2) be relatively low cost, especially so that researchers in developing countries (like Brazil) could replicate the procedure without major financial hardships; (3) it had to be a quick collection, because the tide pool ceases to be a pool from the moment it connects with the rest of the ocean with the rise of the tide, and this takes an average of 3 to 4 hours. So, in order to contribute to the project that was being developed in the Araçá Bay, which was to identify the largest number of fish of various sizes that lived in Araçá, we built a net with mosquito netting, but adapted to catch fish!!!

“Picaré” (beach seining) with mosquito netting - Use of Picaré built with mosquito netting in a sandy pool in the Baía do Araçá.

(Photograph by Marina Brenha-Nunes, CC BY-SA)

Although sandy pools are more labor intensive to collect in the field than rocky pools, mainly due to their enormous size and the short working time available, we were able to achieve a level of 89% capture efficiency with the procedure developed! In other words, if there were 100 fish living in a sandy puddle, we would be able to catch 89 of them. And this is great, because it shows that our protocol was efficient and can be replicated by other researchers, and even improved!

Regarding the species found, we identified some different species between the two types of pools. For example, in the sandy ones we captured many individuals of the Brazilian silverside (Atherinella brasiliensis), Mojarra (species of the genus Eucinostomus sp.), the Darter goby (Ctenogobius boleosoma - which inhabits places with a light bottom), and the Ladyfish (Elops saurus). On the rocks, we basically captured species that have adaptations to tolerate the conditions of a tidal pool, such as high temperatures and salinity, decreased water levels, and oxygen, as was the case of the Frillfin goby (Bathygobius soporator - which inhabits sites with darker bottoms and burrows in crevices in the rocks) and a type of Molly miller (Scartella cristata, which also burrows into cracks). So different from the sandy sites, that experts do not even consider the sandy species as adapted to life in tide pools!

Fish collected from rocky pools. Upper photo: Frillfin goby (Bathygobius soporator); Bottom photo: Molly miller (Scartella cristata).

(Photograph by Pedro Félix, available through FishBase, CC-BY-NC)

Fish collected in sandy pools, the Mojarra (species of the genus Eucinostomus sp.) (Photograph by Carla Elliff, available through FishBase, CC-BY) and the Ladyfish (Elops saurus). (Photograph by Gustavo Guedes, available through FishBase, CC-BY)

You must be wondering, "What do you mean? But then, how were they there?"

The big difference is that we usually found fish species typical of rocky tide pools from juvenile to adult stages, as was the case with Frillfin goby and Molly miller, but in the sandy pools we found many larvae (early stage of fish development - learn more about fish larvae here) or only juvenile individuals of species that we usually find at high tide, as is the case with Brazilian silverside and Mojarra. The difference in the number of species and the amount of fish caught between the rocky and sandy pools is quite striking. This brings up a very important ecological issue for the conservation of coastal environments, showing that we need to know about all tide pool types before proposing unique measures for all types of environments.

Tide pools have always been ecologically important habitats for several species that participate and balance the coastal food chain, and may also interact with reef fish species and serve as bait for fishermen. Putting into question other types of tide pools, with some different functions from those already known, we understand that the pools may also play a role as shelter for the larvae, escaping from some predators and taking the opportunity to feed with greater "tranquility". Many of the larvae found were of species that are captured by fishermen as adults during high tide for trade or their own consumption. But if we degrade these environments, where will these larvae take shelter?

Besides thinking about the future of these larvae, we must also think that along with port construction comes the issue of invasive species (species that are not native to the region). We also detected this scenario in Araçá, once we captured two exotic species (not native), the Omobranchus punctatus (muzzled blenny) and the Butis koilomatodon (mud sleeper) also known as the bellied or sleeper, in sandy pools and which are potential competitors of our native species. In addition to degradation, the environment can be dominated by these species, which unbalance the environment and the life cycles of other species.

As an environmentalist, it is necessary to know, protect, and conserve coastal areas so that we can have a productive return for our lives and for our future generations, thinking about the natural ecological balance of the system, to continue harvesting the good fruits of nature.



BRENHA-NUNES, M. R. (2016). Ictiofauna em poças de maré arenosas e rochosas e seus fatores estruturadores em uma planície de maré subtropical. Dissertação de mestrado. Universidade de São Paulo, São Paulo, p. 79.

BRENHA-NUNES, M. R.; CONTENTE, R. F.; ROSSI-WONGTSCHOWSKI, C. L. D. B. (2016). A protocol for measuring spatial variables in soft-sediment tide pools. Zoologia 33(2), 1-4. Disponível em: <>

CONTENTE, R. F.; BRENHA-NUNES, M. R.; SILIPRANDI, C. C.; LAMAS, R. A.; CONVERSANI, V. R. M. (2015). Occurrence of the non-indigenous Omobranchus punctatus (Blenniidade) on the São Paulo coast, South-Eastern Brazil. Marine Biodiversity Records 8(e73), 1-4. Disponível em: <>

CONTENTE, R. F.; BRENHA-NUNES, M. R.; SILIPRANDI, C. C.; LAMAS, R. A.; CONVERSANI, V. R. M. (2016). A new record of the non-native fish species Butis koilomatodon (Bleeker 1849) for southeastern Brazil. Biotemas 29(2). Disponível em:<>


About Marina Brenha-Nunes

Biologist, graduated from Universidade Presbiteriana Mackenzie (São Paulo, SP). Since school, I have wanted to live the life of a marine biologist, starting with a crazy desire to participate in the Humpback Whale Institute, then TAMAR and went to work with fish from a volunteer internship at the Oceanographic Institute of the University of São Paulo (IO-USP) in 2011. I worked directly with otolith morphology (bones of the inner ear of bony fishes), going through several environmental consultancies and environment studies with private schools. After that, I joined the master's program at IO-USP and finished it in May 2016 with a thesis on fish in tide pools. Now, I have decided to dedicate myself to basic education as a State high school teacher.

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