Ciliates as the crucial food source of larval fish in a shallow eutrophic lake

Abstract

We assessed feeding of planktivorous fish larvae on ciliate protozooplankton in shallow eutrophic Lake Võ rtsjä rv, Estonia to determine whether ciliates are an important part of larval fish diet. We collected larvae of the most common fish species and examined their gut contents by gut segmentation and epifluorescence microscopy. Ciliates were present in the gut of all fish larvae. Gut segmentation analysis showed clear differences in food composition between gut quarters. The hard-bodied food items were quite evenly distributed in the gut, but the soft-bodied ciliates and rotifers were present only in the first gut quarters. Neglecting differences in prey digestion rate leads to underestimation of diet amount and composition and, especially, to underestimation of the role of protists in food. In Võ rtsjä rv, ciliates account for , 60% of the total carbon biomass consumed by fish larvae. The food requirement of fish larvae during first feeding stages (May-Jun) would not be met without consumption of ciliates. Hjort (1914) was first to put forth the so-called 'critical period' hypothesis: the availability of suitable prey at the time when larval fish switch from endogenous to exogenous feeding determine their survival and, thus, the fish yearclass strength. This hypothesis and the further refined theories that derive from it, are all based primarily on the consumption of crustacean prey. Up to date, there are very few studies that have considered the direct protist consumption by fish larvae. Most planktonic protozoa are fragile and therefore get easily degraded in the fish gut. Thus, protist ingestion may have been neglected in the past because of difficulties describing the gut contents of fish larvae, due to a high degree of degradation of the food and the use of normal techniques of microscopy for species identification (Fukami et al. 1999).It has been suggested that most of the coastal waters do not contain enough (metazoan) prey to support larval growth and survival (MacKenzie et al. 1990). Also, some studies indicate that the guts of field-caught larvae often appear empty (van der Meeren and Naess 1993). One explanation for these paradoxes (i.e., apparent low prey levels and empty guts) is that food is sufficiently abundant, but easily digested and therefore not detected by researchers (Figueiredo et al. 2005;Montagnes et al. 2010). One such possible food source that larval fish can ingest and quickly digest is protozoa. If this is the case, the focus on metazoan prey has resulted in fisheries scientists overlooking a key trophic linkage (Montagnes et al. 2010).There have been only sparse field studies concerning protozoa as food of the fish larvae, and most of them have focused only on the hard-bodied protists (Watson and Davis 1989). Only few have tried to estimate the importance of soft-bodied protozoans in the larval diet (Fukami et al. 1999;Nagano et al. 2000;Figueiredo et al. 2005). Modern fluorescence microscopy approaches have been rarely used in this specific research field. Lessard et al...

Biogenic methane contributes to the food web of a large, shallow lake

“…Planktonic ciliates form an especially important food for larval fish in Lake Võrtsjärv (Zingel et al ., ), accounting for c . 60% of the total carbon consumed by fish larvae, and the food requirements of larvae during first feeding stages in May and June could not be met without consumption of ciliates (Zingel et al ., ). Given the exogenous feeding (at the stage of growth when larval fish ingest live food) and the rapid carbon turnover rate of fast‐growing larval tissues (Weidel et al ., ), the low δ 13 C signatures of the smallest larval fish (δ 13 C ranged from −38.4 to −36.4‰) may reflect some use of methane‐derived carbon.…”

“…The most depleted carbon isotope signatures were measured for fish larvae ingesting ciliates and rotifers. Planktonic ciliates form an especially important food for larval fish in Lake Võrtsjärv (Zingel et al ., ), accounting for c . 60% of the total carbon consumed by fish larvae, and the food requirements of larvae during first feeding stages in May and June could not be met without consumption of ciliates (Zingel et al ., ).…”

Towards sustainable exhibits - application of an inorganic fertilization method in coral reef fish larviculture in an aquarium

“…; Zavala‐Leal, Dumas, Pena, Contreras‐Olguin & Hernandez‐Ceballos ; Hagiwara, Wullur, Marcial, Hirai & Sakakura ). Smaller prey such as ciliates and flagellates are used less often, and usually in experimental scale, although studies have revealed that they contribute substantially to larval fish diets in nature (De Figueiredo, Nash & Montagnes ; Friedenberg, Bollens & Rollwagen‐Bollens ; Zingel, Paaver, Karus, Agasild & Noges ). Several rearing experiments also revealed that ciliates and flagellates could enhance the survivorship of fish larvae, especially those with a smaller mouth (Nagano, Iwatsuki, Kamiyama & Nakata ; Tew et al .…”

The trophic role and impact of plankton ciliates in the microbial web structure of a tropical polymictic lake dominated by filamentous cyanobacteria

“…Ciliate are adequate as food sources for first-feeding fish larvae because feeding success is related to non-armoured prey, whereas feeding failure occurs with armoured dinoflagellates because these organisms may pass unaltered through the larval gut (Scura and Jerde, 1977;Lasker, 1981). Ciliates have been found to sustain the survival and growth of first-feeding larvae at other eutrophic shallow lakes (Zingel, 2012), and at Catemaco, they similarly sustain the threadfin shad fishery.…”

“…Plankton ciliates drive biomass from autotrophic and heterotrophic picoplankton to upper trophic levels; therefore, the abundance of autotrophic and heterotrophic picoplankton abundance was simultaneously recorded. Based on their size and other features, plankton ciliates are the most likely food source for first-feeding larvae of fish species that support local fisheries, and because this stage is considered critical in fish life cycles, the maintenance of fish stocks depends on ciliates (Zingel et al, 2012). The selective filter-feeding activity of these fish species is believed to be at least partially responsible for Catemaco's plankton composition (Kormákova and Tavera, 2003).…”