Using Gambusia affinis growth and condition to assess estuarine habitat quality: a comparison of indices

Abstract

Numerous indices have been used to estimate fish growth and condition; however, differences in sensitivity and reliability of the methods have hampered efforts to identify appropriate indicators for routine evaluation of habitat quality in the field. We compared common morphometric (length, weight, somatic growth, length-weight condition) and biochemical (RNA:DNA ratio, relative DNA content, energy density) growth indices on the same wild-caught mosquitofish Gambusia affinis to examine their usefulness as indicators of habitat quality. A laboratory experiment was used to quantify growth rates of wild-caught G. affinis under different feeding treatments. Field studies consisted of both a short-term enclosure experiment (10 d) and weekly (7 wk) fish collections to compare growth indices in managed inflow and reference marshes during a winter/spring freshwater pulse event in upper Breton Sound, Louisiana, USA. Marshes flooded by restored freshwater pulses were capable of producing optimum growth (0.001 g DW d -1; DW = dry weight) and energetically valuable habitat (> 6000 cal g -1 DW) for trophic transport. Because of differences in timing of response, morphometric and biochemical indices were generally not directly correlated, but there was clear agreement in direction and magnitude of response. The most striking difference in timing was that biochemical indices (RNA:DNA) responded more slowly to treatments than did morphometric growth indices. While gross patterns are comparable between indicators, differences in sensitivity and response time between indicators suggest that choice of indicator needs to be accounted for in interpretation and analysis of effects.KEY WORDS: Growth · Energy density · RNA:DNA ratio · Resident species · Poeciliidae · Estuary · Fish · Louisiana Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 412: [231][232][233][234][235][236][237][238][239][240][241][242][243][244][245] 2010 some methods have hampered efforts to identify appropriate indicators for routine evaluation of habitat quality in the field.Increasing threats to fish populations arising from both natural events and anthropogenic activities have led to extensive estuarine restoration activities around the world and a critical need for appropriate indicators to assess the relationship between restored and altered habitat and their associated fish populations (Madon 2008). In particular, restoration in large-river delta environments (e.g. Williamson River Delta, Sacramento-San Joaquin River Delta, Mississippi River Delta) often focuses on the reconnection of the riverine flood events that provide extended access to highquality, ephemerally flooded estuarine habitats for transient and resident fishes (Kneib 2000). These flooded habitats have been shown to be functionally valuable for fish growth due to an abundance of high quality food sources (Madon 2008). In coastal Louisiana, the Caernarvon Freshwater Diversion (Caernarvon) is one such project that restores the annual s...

Effects of seascape context on condition, abundance, and secondary production of a coral reef fish, Haemulon plumierii

“…Examination of RNA:DNA to investigate growth rate is based on the premise that the amount of DNA in cells remains constant, while the amount of RNA increases with growth rate as more protein synthesis is required. This technique has been used successfully as a measure of relative growth in fishes (Buckley 1984, Folkvord et al 1996, Garcia et al 1998, Buckley et al 1999) and has been shown to typically integrate growth rates over a period of weeks (Johnson et al 2002, Piazza & La Peyre 2010.White grunts were collected from artificial reefs using mesh wire traps with mesh sizes of 1 and 2 cm in April 2010. Traps were deployed in the benthos surrounding the artificial reefs and were allowed to soak for 2 to 14 h before being retrieved.…”

Influence of Seascape on Coastal Lagoon Fisheries: the Role of Habitat Mosaic in the Venice Lagoon

“…Size of commercial species is of particular importance for fisheries, since larger specimens are usually more valuable. Body condition measures in fish are commonly used as indicators of ecosystem health in transitional waters, being employed to evaluate productivity and anthropogenic pressures among habitats (Amara et al 2007;Piazza and La Peyre 2010;Cavraro et al 2019). This work emphasises that fish individual weight in transitional waters can be influenced by morphology and seascape structure, these being potential drivers of fish individual health.…”

Measuring Changes in Consumer Resource Availability to Riverine Pulsing in Breton Sound, Louisiana, USA

“…At the same time, other studies in this system demonstrate that these riverine flood pulses deliver a significant amount of basal resources (140 µM of total N, 5 µM total P) at a level that is directly comparable to other allochthonous resource pulses in aquatic habitats (migratory salmon, [8]; migratory waterfowl, [44]; cicada carcasses, [9]) and that these nutrients are rapidly assimilated in the upper estuary [24]. Studies in this estuary also show that riverine nutrients propagate into the resident nekton consumers [25]–[26], [28]–[29] supporting the bottom-up control concept. However, we did not quantify the actual transfer of this subsidy to the higher-order consumers (i.e., predation).…”

“…These resources are readily assimilated by secondary consumers [12] , [25] – [28] and may be responsible for increased nekton growth, changes in community structure, and trophic diversity [28] – [31] . Riverine pulses are particularly important for recruitment of resident nekton consumers that key into these flood events for spawning and rapid growth [21] , [29] – [30] , [32] .…”