Mercedes Ledesma
Marine Ecology 425
Chris Steffen
10/4/2018
Identifying Percent Mortality Between Blue Runner (Caranx crysos) and
Sargassum fish (Histrio histrio) in the Northern Gulf of Mexico
Abstract
Sargassum can be a very beneficial species of algae to both the environment and to other
animal species. In this case Sargassum algae have become a perfect habitat for both the
Sargassum fish (Histrio histrio) and the Blue Runner fish (Caranx crysos). The objective would
be to decipher which species would benefit the most from this habitat by calculating highest
mortality rate. It is suggested that the Sargassum fish would better benefit due to their
camouflage like bodies which perfectly resemble that of the algae. Counter to that, the blue
runner fish also contain a great advantage because they would use the sargassum to aid in
predation. With a 1m x 2m frame surface neuston net, Blue Runner and Sargassum fish were
collected and later measured in the lab to determine the standard lengths within both species.
After determining the length frequency, hatch date distribution and age frequency, the percent
mortality rate of both species can be calculated. The mortality rate was higher in Blue runner
than in the Sargassum fish meaning the Blue runner are highly vulnerable in the sargassum
habitat then the Sargassum fish would be. The sargassum fish has a greater advantage in using
this particular habitat for both protection and foraging grounds.
Introduction
In various pelagic species, there is a distinct rate in which each undergoes natality and
mortality. Depending of the species there can be a very high natality rate (r-strategists) which in
turn would consist of a high mortality rate. This crucial time is seen in the larval stages, any time
from the species first hatch period to the first time they feed (Voss et al, 2001). Over 99% of
many pelagic larvae experience a high mortality rate in under a month. This extremely high rate
is due to numerous factors such as predation, environmental conditions, disease and even
starvation contributing to their survival.
Sargassum is a type of brown macroalgae or also known as a seaweed in the family
Sarcassaceae. Within the Atlantic and the gulf, the two main pelagic species are Sargassum
natans and Sargassum fluitans (Parr, 1939). They are typically seen as a free-floating species
Mercedes Ledesma
Marine Ecology 425
Chris Steffen
10/4/2018
inhabiting the oceanic water column. Typically, in the Atlantic Ocean, sargassum creates a
perfect habitat for a diverse group of species such as turtles, invertebrates and a wide range of
fish (Casazza and Ross, 2008). Two examples would be the Sargassum fish (Histrio histrio) and
the Blue runner fish (Histrio histrio). Both of the species utilize the free floating sargassum to
their own benefit. Whether its for protection or for foraging the species within and around the
algae, the sargassum is a perfect habitat for the two species.
This profiting habitat is so physically intricate that it creates the perfect area for scientists
and fishery managers to take an interest for study (Wells and Rooker,2004). Since so many
species can utilize the same habitat, it’s difficult for either to properly thrive unless there is one
dominant species. So, the question remains, which species (Blue runner or Sargassum fish) will
have the highest natural mortality rate within the sargassum habitat. It can be suggested that the
sargassum fish will have the lowest natural mortality rate based on the fact that they physically
resemble the sargassum algae and can easily be undiscoverable from predators. On the other
hand, blue runner fish can also have a low rate because they would possibly utilize this habitat to
capture their prey due to their larger physique. A daily percent mortality rate can be determined
by collecting the standard lengths of numerous sargassum and blue runner fish.
Materials and Methods
On June 9, 2012, fish larvae were collected the Northern Gulf of Mexico. A neuston net
(1m x 2m frame) was towed at the coordinates 26-28°N and 86-93°W and each tow lasted about
10 minutes. Once these fish samples were collected they were taken to the lab to be measured
individually.
Each individual specimen from both the Sargassum fish (Histrio histrio) and the Blue
runner fish (Histrio histrio) species were measured for their standard lengths (mm) starting from
the tip of their mouth to the end of their caudal peduncle. Once every single fish was measured
they were placed in a data sheet to use for later determining the mortality rate.
Within excel, length frequency, hatch date distribution and age frequency were calculated to
eventually determine the mortality rate. The length frequency histogram was created in 5 mm
increments and included the standard lengths for both species and a length range starting from 0
to the highest possible length. Using the growth rate equation (Age = (SL- X.X)/X.X) and the
Mercedes Ledesma
Marine Ecology 425
Chris Steffen
10/4/2018
standard length, each individual age of each fish can be determined. Next the Hatch date of each
fish was calculated by subtracting the precalculated age of each fish from the date all the fish
were caught, which was 6/9/2012. With this information a hatch-date distribution can be made.
The bins for the hatch date distribution is formatted in single day increments beginning from the
first hatch date to the last. This ensures accuracy and displays each individual hatch date. These
numbers will aid in determining the age frequency distribution which will range from the
maximum fish age to zero. The histogram will also be made in single day increments within the
bins while the major interval is in a ten-day span. Now that the age frequency distribution has
been created, the highest data point in the histogram can be determined. This highest peak
represents where there is the highest abundance of each species. From that highest point, nine
