The fish that defines the Venice Lagoon is the Mediterranean killifish, Aphanius fasciatus, a small species that spends its entire life in brackish water instead of coming in from the sea. Thirteen months of monthly netting in one natural salt marsh on the northern margin of the lagoon, from May 2023 to May 2024, caught it in every month except December and January (Altavilla et al., 2026).
That study is the first to describe the barene, the lagoon's salt marshes, from the inside. The marshes are cut by a branching network of small tidal creeks that flood and drain on a semi-diurnal tide, so every animal inside them is pushed out twice a day. The creeks in the study area ran about 27 to 230 metres long and 0.19 to 3 metres wide across roughly 0.5 square kilometres, and the fish were not spread evenly through them.
This page explains which fish the lagoon holds, how a marsh creek works as a piece of machinery, which creeks the killifish actually chooses, why the species is legally protected, when in the year it is present, and what the finding means for the marshes Venice is trying to rebuild.
What fish live in the Venice Lagoon?
The lagoon's signature fish is the Mediterranean killifish, Aphanius fasciatus, a small cyprinodont first described in 1821. It matters out of proportion to its size because it is a resident: it completes its whole life cycle inside transitional waters, so its numbers reflect conditions in the lagoon itself rather than what is happening offshore (Altavilla et al., 2026).
It is far from alone. Monthly netting in the marsh creeks also took gilthead seabream (Sparus aurata), the Adriatic dwarf goby (Knipowitschia panizzae), mullets of the genus Chelon, the big-scale sand smelt (Atherina boyeri) and the green crab Carcinus aestuarii, which Venetian kitchens know as moeche and masanete. Two of those names will be familiar from a fish counter rather than a field guide.
The honest caveat belongs in the same breath. The authors list those species with no counts and no proportions at all, and state plainly that species co-occurrence was beyond the original scope of their study. The idea that the killifish could work as an umbrella species, so that protecting its creeks protects the rest, is offered as a hypothesis for future testing and not as a result (Altavilla et al., 2026).
How a salt marsh creek works
A barena is not a flat island. It is drained by a hierarchy of channels: one main creek, which the study calls the town creek, connects three first-order creeks, and each of those carries second-order and third-order tributaries. The fieldwork covered the three first-order creeks and eight second-order creeks, each of the latter serving as one sampling station, over an area of about 0.5 square kilometres (Altavilla et al., 2026, p. 3).
The scale is domestic. Creeks ran from about 27 to 230 metres long and from 0.19 to 3 metres wide. What drives them is the semi-diurnal tide of the northern Adriatic: at high water the creeks fill and give a range of aquatic organisms access to the marsh, and on the ebb those animals are forced to leave again (Altavilla et al., 2026, pp. 2 to 3). That twice-daily flush is the thing a visitor can watch from a boat.
Twenty-two features of the creeks were measured, in the field or from aerial imagery: width taken at bank-full level every 10 metres from mouth to head, bottom slope at the central point every 10 metres, and bends counted wherever the main channel changed direction by more than 30 degrees.
Which creeks hold the most fish
The result reads like a specification. The killifish concentrates in creeks that keep some water at low tide, that fill and empty slowly, that drain a large area of marsh, and that carry plenty of bends. It thins out in creeks that drain flat and empty, and in creeks that are steep or excessively twisting (Altavilla et al., 2026, p. 6).
Each preference has a reason attached. A creek that never fully empties is a refuge from predators. More bends mean greater structural complexity, which means more microhabitats and more places to hide. A larger drainage area delivers more nutrient input from the land, which feeds the bottom of the food chain. Clearer water suits a fish that hunts by sight, and more chlorophyll in the sediment, used as a proxy for the microscopic algae living on the mud, means more food reaching it.
One correlation in the paper is a season in disguise, and the authors say so. Killifish numbers rise as dissolved oxygen falls, which looks like a preference for poorly oxygenated water. It is not. Oxygen declines through spring and summer as the water warms, and the fish are most abundant in the warmest months, so temperature is doing the work (Altavilla et al., 2026, p. 4).
Is the Mediterranean killifish protected?
Yes, twice over. The species is listed in Annex II of the European Union Habitats Directive (92/43/EEC) and in Appendices II and III of the Bern Convention of 19 September 1979, and its conservation status is assessed as unfavourable-inadequate, which the authors attribute largely to the loss of the habitats it depends on. The Habitats Directive treats it as a resident species requiring the establishment of special conservation areas, precisely because it is tied so tightly to particular shapes of water: salt marshes, small creeks and shallow pools (Altavilla et al., 2026, p. 2).
Its range is the central and eastern Mediterranean, with established populations in the Suez Canal and the Red Sea, and according to the IUCN its distribution is limited to coastal areas.
The survey method was built around not killing it. A fyke net was set at the mouth of each creek at the peak of high tide and left in place until the ebb finished, so the draining water delivered the catch. The fish were photographed against a millimetric sheet, identified and measured on screen, then released. The work was carried out under Italian animal welfare law, through Article 18 of regional decree 54/2012, and supervised by a certified veterinarian.
When the fish are there, and what the water is like
The killifish was caught in the marsh in every month of the thirteen-month survey except December and January, when water temperature fell to between 3 and 10 degrees C, and abundance peaked in the warmest months (Altavilla et al., 2026, p. 4). Late spring and summer are when the creeks are busiest.
The paper publishes no salinity value and no creek depth for the marsh, although both were measured, so nothing here can be said about how salty that particular water is. The nearest measured figures come from a separate year of sampling around Chioggia, in the southern lagoon: full-year average salinity of 30.19 and 30.66 PSU at two stations inside the lagoon, against 33.25 PSU on the open Adriatic coast at Pellestrina and 5.64 PSU at the mouth of the Brenta, with water temperature running from winter means near 11.9 degrees C to a summer mean of 27.11 degrees C at the coastal station (Boschiero et al., 2026).
Those stations sit tens of kilometres south of the marsh in the killifish study, so treat them as the scale of the gradient rather than as readings from the same place. The same work measures the lagoon at about 550 square kilometres and names the traditional local gear, the tresse con cogolli.
What this means for rebuilding the barene
Venice has been losing marsh for a century. The killifish paper reports, citing earlier surveys by Silvestri and colleagues in 2003 and Sarretta and colleagues in 2010, that the lagoon's salt marshes fell from 149 square kilometres in 1912 to 37 square kilometres in 2003, mainly through erosion caused by human activity, with further losses predicted. Rebuilding them is now a legal obligation as much as an ecological one: the EU Nature Restoration Law (Regulation EU 2024/1991) sets a binding target of restoring at least 20 per cent of the Union's land and sea areas by 2030, and restoration measures for all degraded ecosystems by 2050.
The practical advice the authors draw from their own fish is specific. Build high-volume creeks, set them far enough apart to maximise the area of marsh each one drains and the nutrients it collects from the land, and make them winding and slow-flowing so that they do not fully empty at mean low tide (Altavilla et al., 2026, p. 8).
They add their own limit. A single resident species gives a robust proxy for local conditions, and it does not capture how every other group of organisms would respond, so these recommendations should be read as context-dependent and indicative rather than universally prescriptive.
