The effects of hypoxia on the behavior and development of larval estuarine fish
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Abstract
Hypoxia has been proven to severely impair fish survivorship, cause alterations to certain
behaviors, and induce embryonic deformities. Despite this, the effects of hypoxia on many areas
of larval behavior, such as anxiety-related behaviors and light/dark preference, are not well
known. Therefore, I studied its impacts on larval spotted seatrout (Cynoscion nebulosus), red
drum (Sciaenops ocellatus), and striped bass (Morone saxatilis) behavior by exposing them to
either normoxic or hypoxic conditions. I received four separate shipments of both sciaenid
species and two shipments of striped bass. Each shipment corresponded to a separate trial that
included three replicates each of normoxia and hypoxia. All fish were acclimated and maintained
in six 19-liter aquariums for 24 hours. After acclimation, I utilized ten fish from each aquarium
in scototaxis (light/dark preference) testing and ten from each aquarium in novel object testing
for a sample size of 240 (120 for striped bass) fish for each behavioral test across all four trials.
Scototaxis and novel object protocols were used to examine behavioral changes. Both behavioral
tests lasted 180 seconds since it was found larvae “froze” in movement past this time. Once
behavioral tests concluded, all tested fish were euthanized and observed under a microscope.
ImageJ software was used to check for possible eye deformities that may have led to alterations
in visual acuity that could affect behavior. Standard lengths were also taken to determine
hypoxia caused impairments to growth rate. In the scototaxis test, seatrout preference was for the
light side in normoxia (p = 0.00104) and no preference between light and dark under hypoxic
conditions (p = 0.22628). Red drum showed no preferences to light or dark in either normoxia (p
= 0.44139) or hypoxia (p = 0.17702). Striped bass showed a strong light preference (p <
0.00001) that did not change after hypoxia exposure (p = 0.00022). In the novel object test,
hypoxia-exposed seatrout spent significantly more time in the inner (p = 0.00318) and middle
rings (p = 0.00031) compared to normoxia-exposed individuals. Red drum (p = 0.11876) and
striped bass (p = 0.92828) showed no significant change in behavior in hypoxia compared to
normoxia. None of the three species showed any changes in eye development in terms of eye
width and area. There was no significant difference between the left and right eyes either. In
terms of standard lengths, the only species that showed a significant difference between control
and treatment was red drum (p = 0.02167); hypoxia-exposed individuals were statistically
smaller than normoxia exposed ones. Based on my statistics analysis, I can conclude that spotted
seatrout are behaviorally influenced by hypoxia exposure whereas red drum are developmentally
affected. Striped bass showed impairments to neither behavior or development, but many
confounding factors in the testing of this species calls into question the validity of these results.
Spotted seatrout behavioral changes likely did not stem from eye deformities, but rather changes
to mechanorecptor sensory development. Future testing should look into hypoxia-caused
alterations of mechanoreceptor systems within coastal fish species. Further behavioral testing
should utilize both camera tracking software along with physical observation.