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This study investigates the impact of Undaria pinnatifida on the reef fish assemblage associated with the coastal rocky reefs of Golfo Nuevo in northern Patagonia, Argentina. Undaria pinnatifida is a laminarian kelp, indigenous to the temperate regions of Japan, China and Korea. It has spread around the world by international shipping and has also been introduced in some regions for mariculture. Its expansion into the northern Patagonian gulfs and Peninsula Valdes, a UNESCO Natural Heritage Site, may threaten eco-tourism activities that depend on local biodiversity and the esthetic quality of the ecosystem. In particular, there is concern about the impact of Undaria on a variety of recreational activities including diving, angling and spear fishing, which have been conducted historically in the gulfs typically concentrated on rocky reefs and their associated fish fauna. Reef stakeholders were the first to alert about the radical transformation of the sea bottoms induced by the presence of Undaria and its possible impacts on the native flora and fauna. This study was initiated in this context to evaluate the effects of Undaria on the patterns of reef occupancy and abundance of the most characteristic reef-dwelling fish species, and on the benthic macrofauna on which they prey. The study included four fish species that are relatively sedentary (“salmón de mar” Pseudopercis semifasciata, “mero” Acanthistius patachonicus, “turco” Pinguipes brasilianus, and “escrófalo” Sebastes oculatus) and three schooling species (“papamoscas” Nemadactylus bergui, “besugo” Pagrus pagrus and “sargo” Diplodus argenteus). Two subsidiary objectives were to gather basic information on the population dynamics of Undaria in Golfo Nuevo, and to quantify the sources of variability in the estimates of reef fish abundance obtained by underwater visual census. Sampling of three reefs over two and three years with an approximately monthly periodicity indicated that Undaria follows a strong seasonal cycle. Young sporophytes recruit in winter and subsequently grow rapidly to form dense forests by spring and early summer. Fronds start to decompose in late spring, and their size and weight decrease progressively toward mid-autumn when reefs become free of algae. During late spring and early summer many large plants detach from the substrate and drift with the currents covering irregularities of the sea floor. Given this strong seasonal pattern, the sampling design used to evaluate the impact of Undaria on fish reef occupancy and abundance had to consider not only the precision of fish count data obtained by visual census but also the level of variability associated with different temporal scales. Among the temporal scales considered, the inter-annual variation associated with seasonal fluctuations in fish abundance was the largest in magnitude. The instant variation due to positive or negative responses of fishes to the divers, and to fish displacements across the boundaries of the sampled sections of the reefs at very short time scales (e.g. seconds to minutes) was the most important in terms of limiting the accuracy of the data collected by visual census. The effect of Undaria on the abundance and temporal dynamics of reef fishes was evaluated by monitoring fish abundance in reefs with and without Undaria over two or three annual cycles. A total of seven reefs were monitored, three with Undaria and four without, the latter being colonized during the study period. In addition, site fidelity and residence time of the mero A. patachonicus were studied in reefs with and without Undaria. No effect of Undaria was detected on the abundance patterns of the species studied, except for a negative impact found on two reefs characterized by their low relief. The hypothesis that there is a differential effect of Undaria depending on the reef relief was experimentally evaluated. Fish abundance was shown to decrease markedly in lowrelief reefs that had been covered by Undaria during spring, and not in those that were free of Undaria. This impact involves a transitory habitat loss for reef fishes inhabiting low-relief reefs during late spring and early summer. Fish re-colonized the reefs during late summer and autumn when they became again free of Undaria. The effect of Undaria on the abundance and availability of benthic macrofaunal prey was evaluated by a removal experiment and by focal behavioral observations. Prey abundance and macrofaunal diversity was found to be higher when Undaria was present than when the sporophytes were removed. This effect is attributed to the provision of new habitat by Undaria, a larger and structurally more complex seaweed than the local native species. Behavioral observations indicated that preys were available to reef the fish species studied. Finally, although the impact of the documented effect of Undaria on reef fish populations is unknown, the transient habitat loss could have a negative effect on a variety of recreational activities including diving, angling and spear fishing concentrated on rocky reefs and their associated fish fauna. On the other hand, the impact of Undaria on benthic macrofauna enhances prey offer to reef fishes and this could have a positive effect on reef fish populations, for example by increasing the carrying capacity of rocky reefs and the survival of juveniles and adults.
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