The beach may seem like a lifeless environment, as we look out at the endless sand and sea. But if we pay a little attention, we start to notice the crabs, mole crabs, and sand dollars, and our opinion starts to change. Then we go for a swim, and, sometimes, even when in shallow waters, it is possible to spot some fish. The number of fish sighted may seem minimal when compared to what can be seen when swimming close to a coral reef or a rocky shore. However, even without any craggy rocks, holes in corals, or mangrove roots to protect them, it is possible to spot some fish “fighting” with the ebb and flow of the waves.
And it was exactly this group of fish that I decided to study during my master’s degree, the fish of the shallow zone of beaches. The main objective of the study was to analyze the influence of several environmental variables (different beaches, tides, seasons, salinity and temperature) on the composition and structure of the ichthyofauna (meaning fish fauna) from beach environments. To obtain the data for my project, I collected with the help of technicians from the IOUSP Research Base in Cananeia (SP) fish from three beaches situated in Ilha Comprida and three in Ilha do Cardoso, southern São Paulo State (that is, I had the opportunity to travel during one year to a special place, as said in the post “Internacionalizar é preciso”). These beaches had different levels of exposure to the estuary waters: two facing the interior of it, where almost no waves were observed, two intermediate, and two exposed, with bigger waves and no influence of estuarine waters. Samples were taken using a net called a seine, which is manually dragged by two people, one on each side.
Sampling with the seine net in an “exposed” beach and in a “sheltered” one in Ilha Comprida (SP). (Photographs by Jana M. del Favero, licenca CC BY SA 4.0).
A total of 57 species were sampled, mostly in their juvenile phases, varying from 0.4 to 6 cm in length. Some larvae were sampled too (see post “A vida “dura” de um peixe marinho bebê”). The more protected the beach, the greater the quantity of sampled individuals and greater the number of species. On the other hand, the more exposed beaches showed high dominance of a few species, mostly Pompanos (genus Trachinotus) and Mullets (genus Mugil).
Trachinotus carolinus (Pompano), left (A and B); and Mugil curema (Mullet), right (D and C). The photos on the top show them young, while the bottom ones show them as adults. (Photos A and D are by Jana M. del Favero, licence CC BY SA 4.0 / Photo B is by Trevor Meyer, licence CC BY NC / Photo D is by Carla Elliff, licence CC BY).
Fish use these beaches for many reasons: as growth areas (despite the lack of hiding spots, the simple fact that this area is shallow prevents larger predators from feeding there, offering protection to the small fish); as a migration route between the ocean and the estuary; and only one species, Silverside “The Kingfish” (Atherinella brasiliensis) can be classified as a lifelong resident, meaning that it lives there throughout all phases of its development (larvae, young and adults) in the same region throughout the year.
Menticirrhus littoralis, an example of species that uses the area of study for growth (even the biggest individual in the picture was still a juvenile). (Photo by Jana M. del Favero, licence CC BY SA 4.0).
A fish from California, USA, which belongs in the same Family as our “Kingfish” (Atherinopsidae), uses beaches in a quite different way: they go completely out of water to spawn in the sand. The most interesting is that they leave the water by the thousands, so the beach is filled with “dancing” fish. It is worth watching the video:
Another interesting fact is that the largest number of fish and the greatest diversity is obtained during the summer, the season in which the beaches are impacted most by tourism, and also the time that the beach sees the most trash!
Are you interested? Click here to access my dissertation!
Have a great time at the beach, everybody!