Summary
Through manipulations of sensory functions, researchers have evaluated the various mechanisms by which migratory fish, particularly in lotic systems, locate natal spawning grounds. Comparatively less work has occurred on the ways by which fish in lentic systems locate spawning sites, and more specifically, the ways by which displaced fish in these systems locate their broods post spawning. The primary goal of this research was to determine the sensory mechanisms used by nesting, male Largemouth Bass to navigate back to their brood following displacement. This was accomplished by comparing the ability of visually impaired, olfactory impaired and geomagnetically impaired individuals to return to their nests after a 200 m displacement, relative to control males. All treatments were designed to be temporary and harmless. We analyzed the data using a generalized linear mixed model, and found that the probability of an olfactory impaired individual returning to his nest within a given time interval was significantly lower than the probability of a geomagnetically impaired individual returning. Overall, it appears as though olfaction is the most important sensory mechanism used for homing in Largemouth Bass.
Methodology
Data for this study were collected between May 11 and May 15, 2014 on Lake Opinicon in southeastern Ontario. Largemouth Bass nests containing newly-spawned eggs (<3 days old) were located in littoral regions by snorkel surveys and marked with polyvinyl chloride tags for future identification. The mating success of the male was determined by ranking the quantity of eggs in each nest on a scale from 1 to 5, with 1 representing a very small brood size and 5 representing a large brood (Kubacki, 1992, Philipp et al., 1997, Suski et al., 2002).
Nesting males (with the exception of the non-angled controls) were then angled off of the nests by a snorkeler and brought to a nearby boat to be measured (total length) and treated. Each male was randomly assigned one of the following treatments:
a. Non-angled control: Non-angled control fish were not removed from their nests. Nests were simply located, and tagged like all other nests. Since an exact measurement could not be made, total length was estimated by the snorkeler to the nearest 2 cm.
b. Catch-and-release (C&R) 5 m Control: All individuals in this group received simulated treatments for all three impairments involved in the study. The eyes were carefully padded dry with paper towel and gently rubbed for approximately 20 s (the same amount of time it took to treat visually impaired fish). A blunt-tipped 16 gauge needle was used to blow air through the nares. A strip of Fixodent® was applied above the cranial region. The fish were then released 5 m from their nests.
c. Catch-and-release (C&R) 200 m Control: Fish were treated in the same way as the catch-and-release (5 m) control fish, but were released approximately 200 m away from their nests.
d. Visual Impairment (Fig. 1A): Vision was impaired by drying the eyes with paper towel and applying a thin layer of Vaseline®, followed by a layer of pink, opaque Fixodent®. Based on observations made during preliminary trials, this method effectively blocked the vision of the fish for four to eight hours, after which time the Fixodent® began to flake off. There were no noticeable physical or behavioral impairments following eye treatments during laboratory trials. A variety of other approaches were attempted in preliminary trials, including the application of several brands of toothpaste, a mixture of toothpaste and Vaseline®, a mixture of Vaseline® and flour, opaque eye patches held in place by Fixodent® and a layer of Fixodent® with no Vaseline®. Out of all of the attempted approaches, the application of Vaseline® and then Fixodent® on dry lenses was most successful in terms of forming an eye patch of appropriate consistency to remain on the eyes for an extended, but temporary period of time. Treated fish were then transported 200 m away from their nests and released.
e. Geomagnetic impairment (Fig. 1B): A strong bar magnet was fixed between the eyes of the bass, above the cranial region, using Fixodent®. Since strong bar magnets produce stronger magnetic fields than the Earth’s natural field, placing a magnet above the fish’s brain should compromise magnetic navigation (Avens and Lohmann, 2003). After the application of the magnet, fish were released 200 m away from their nests.
f. Olfactory impairment (Fig. 1C): Olfaction was occluded by using a blunt-tipped 16 gauge needle to inject Vaseline® into the anterior nares until it came out of the posterior nares, indicating a full olfactory chamber (Jahn, 1969, LaBar, 1971). Again, olfactory impaired (anosmic) individuals were released 200 m away from their nest.