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The Effects of Light Intensity and Photoperiod on Marine Fish Larvae
Angler fish larvae
Through extensive review of laboratory-conducted studies and quality research, much data has been compiled regarding the effects of light on marine fish larvae. Light intensity, spectrum, and photoperiod have all been found to have significant effects on fish larvae. This paper will discuss the effects of photoperiod and light intensity. These effects vary based on the species and larval stage being studied, but some general concepts can apply to certain variables. Light affects the growth, development, survival, activity, and feeding ecology of marine fish larvae (Batty 1987, Peña et al. 2004, Puvanendran and Brown 2002). Thus, having valid data on the effects of light on fish larvae is of great importance in an aquaculture setting. The majority of marine fish larvae are visual predators and rely on photoreception to determine the location of prey (Peña et al. 2004, Puvanendran and Brown 2002). Pelagic larvae and their prey also rely on photoreception to determine their distribution in the water column, thus affecting their ability to feed. The photoperiod and intensity of light is especially critical during the first feeding period when exogenous feeding begins. At this time larvae either find and consume enough nutrition, or face starvation. There are oftentimes high mortality rates reported during this stage, so anything to decrease these rates would prove beneficial (Peña et al. 2004).
Peña et al. (2004) performed a study that detailed the effects that various levels of light intensity had on the first feedings of Paralabrax maculatofasciatus (spotted sand bass). Four different levels of light intensity were used: 0, 100, 400, and 700 lx. It was found that light intensity has a significant effect on the feeding habits of P. maculatofasciatus at this larval stage. The feeding incidence, which was measured by the percentage of larvae with prey in the digestive tract, increased in conjunction with light intensity. A significantly higher feeding incidence occurred at both 400 lx and 700 lx than at 0 lx and 100 lx. It is important to note that there were no significant differences in the feeding incidences at 400 lx and 700 lx, implying that there may be a maximum level of light intensity at which any increase will not coincide with an increase in feeding incidence. Alternatively, there may be a maximum level of feeding incidence that this species will achieve under these conditions. Despite this, the presence of light in general seems to have some benefit to the feeding incidence of P. maculatofasciatus because larvae in all levels of light intensity had a significantly higher feeding incidence than the larvae present in darkness (0 lx) (Peña et al. 2004).
Peña et al. (2004) also measured feeding intensity by recording the number of prey in the digestive tract. Although larvae in all light intensities had consumed about the same amount of prey, those which were kept in 0 lx rearing tanks showed a significantly lower feeding intensity (Peña et al. 2004). This shows that even a small amount of light is beneficial to the feeding habits of P. maculatofasciatus, whereas having no illumination at all has a negative effect on both feeding intensity and feeding incidence. Puvanendran and Brown (1998) found similar results with Gadus morhua (Atlantic cod) from the Northeast Grand Banks during the first feeding stage. Feeding intensity of these larvae increased with increased light levels. Though 0 lx was not a variable in this experiment, feeding intensity was measured at 8.5 lx for low light and 680 lx for high light. G. morhua larvae from this region were able to more successfully encounter and consume prey under high light conditions than low light conditions (Puvanendran and Brown 1998).
Similar results have been found by during other studies. Downing and Litvak (1999) found that Melanogrammus aeglefinus (haddock) larvae grew better in the presence of higher light intensity. Puvanendran and Brown (2002) found in a later study that an increase in light intensity coincided with an increase in prey capture and growth by G. morhua from the Grand Banks region. Barahona-Fernandes (1979) found that Dicentrarchus labrax (sea bass) larvae showed better growth at higher light intensities as well. When species can find and capture prey more effectively, they grow faster and tend to develop and survive better.
One possible reason for an increase in feeding incidence and intensity coinciding with an increase in light intensity is related to the functionality of the visual system of these marine fish larvae. As stated previously, the majority of marine fish larvae are visual predators (Peña et al. 2004, Puvanendran and Brown 2002). Most of these larvae have a pure cone retina at hatching time and rods are not added to the retina until the larvae have grown sufficiently. Rods are needed to facilitate vision under low light conditions, thus newly hatched larvae, including those in the first feeding stage, have reduced visual capacity at low light levels (Peña et al. 2004, Puvanendran and Brown 2002). Because of this, younger larvae may have higher light requirements than older larvae that have more advanced visual systems (i.e. more rods) (Puvanendran and Brown 2002). Thus if there is not enough visual stimulation due to the absence of light, it may be that larvae cannot differentiate prey clearly (Peña et al. 2004). Blaxter (1986) stated that teleost fish have a visual stimulation threshold of 0.1 lx, so if there is any less light intensity than this, the larvae are not able to properly locate prey. This could explain why larvae performed so poorly under 0 lx illumination. The increase in feeding incidence and intensity with increased light intensity could thus possibly be attributed to an increase in contrast between the prey and the background perceived by the larvae (Peña et al. 2004). This meaning that the higher the light intensity, the more easily seen the prey items are to the fish. It was also discovered that the availability of light during the early life stages of fish affects the development of the eyes. Studies have been performed with Haplochromis burtoni and Salmo gairdneri which proved that light deprivation during early larval stages causes the eyes to develop abnormally, negatively affecting the visual acuity of the fish (Rahmann et al. 1979, Zeutzius and Rahmann 1984). This improper visual development would most likely negatively affect the rest of the fish’s development as well. Larvae that cannot properly identify prey or avoid predators due to decreased visual acuity would have decreased survival and growth rates.
An important observation to keep in mind, which was also noticed by Peña et al. (2004), is that there is sometimes a maximum amount of light intensity that should be presented to certain larvae. Results from the study performed by Peña et al. (2004) show that P. maculatofasciatus larvae presented with 400 lx illuminations and in 700 lx illuminations performed in relatively the same manner. Batty (1987) performed a study that observed the effect of light intensity on activity and food-searching of Clupea harengus (herring) larvae. He found that light intensities that were over 10 lx caused a decrease in swimming speed. This negatively affected how much distance C. harengus larvae covered per unit of time. When this value decreases, food searching becomes less effective (Batty 1987). Batty (1987) also observed that food searching by C. harengus is most effective at around 5 lx. The implication here is that if light intensity is too high, and food searching becomes negatively affected, then as a result larvae take in less nutrients and their growth also becomes negatively affected. Special attention must be given to the specific light requirements of each organism.
The responses to each characteristic of light are specific to each species and the environments they come from, and sometimes, specific to certain larval stages. Yolk-sac larvae are obviously not affected by light in the aspects of foraging or even as significantly as later larval stages are in the aspect of growth. This is because their nutrient source (yolk) is not dependent on visual stimuli. Light is often necessary for the visual development of larvae though, as previously stated (Rahmann et al. 1979, Zeutzius and Rahmann 1984). Batty (1987) observed that the activity levels of both yolk-sac larvae and first feeding larvae were affected by light intensity. At light intensities below the visual threshold, the amount of time spent swimming was considerably less than at intensities above the feeding threshold. At light intensities below the feeding threshold, C. harengus larvae were only active for 6% of the time. Batty (1987) also observed that older larvae had a higher increase in activity levels under the same light intensity conditions as younger larvae (Batty 1987). This increase in activity results in an increase in foraging efficiency because more area is covered per unit of time, and thus the prey encounter rate increases. Puvanendran and Brown (2002) found a significant difference in the prey capture success of G. morhua larvae reared in lower and higher light intensities. Larvae in higher light intensities captured prey much more efficiently, implying that reduced light intensities negatively affect the ability of certain larval species to detect prey, their reactive distance, encounter rate, and searching ability (Puvanendran and Brown 2002). This increase in foraging ability with higher light intensity would enable larvae to grow faster and survive better due to a superior ability to obtain nutrition.
Alternatively to previously discussed species, sometimes having a comparatively higher light intensity can negatively affect marine fish larvae while a lower light intensity offers benefits. Bolla and Holmefjord (1988) found that Hippoglossus hippoglossus (Atlantic halibut) yolk-sac larvae develop abnormally in the presence of light. Puvanendran and Brown (1998) performed a study that differentiated the effects of light intensity on the foraging and growth of G. morhua populations from two different environments. Though G. morhua from the Northeast Grand Banks were positively affected by a higher light intensity, those from the Scotian Shelf were negatively affected. A light intensity of 8.5 lx was representative of low light conditions, and 680 lx was used to represent high light conditions. G. morhua larvae from the Scotian Shelf reared in high light conditions grew to a significantly smaller length than those reared in low light conditions. They also foraged more frequently under low light conditions, which positively correlate with growth rates for G. morhua of this region. Interestingly, G. morhua from the Scotian Shelf did not even survive past the fourth week of rearing under high light conditions. In contrast, the larvae collected from the Northeast Grand Banks performed significantly better in high light conditions, so it was not that these conditions were unsuitable for larvae in general (Puvanendran and Brown 1998). This difference in reaction to light intensity is most likely attributed to the different conditions of the environments that the organisms were collected from. Light intensity and photoperiod change with latitude, which has some effect on larval growth and survival (Suthers and Sundby 1996). G. morhua from the Scotian Shelf region spawn during winter, so their larvae experience relatively low light levels. It makes sense that they then grow and survive better at lower light intensities. On the contrary, G. morhua larvae from the Northeast Grand Banks region spawn during the summer, and their larvae are exposed to higher light levels, thus they seem to grow and survive better in higher light intensities (Puvanendran and Brown 1998). Organisms adapt and evolve to the different environmental constraints that they experience.
On a similar note, Puvanendran and Brown (1998) also found that even though foraging abilities of G. morhua larvae from the Grand Banks region were higher in high light intensity conditions, they continued to feed in light conditions as low as 8.5 lx. There may be a middle ground for each species in which both feeding rate and energy use are at reasonable levels so that costs can remain lower without significantly affecting growth rates. This is a possibility for future studies.
It has also been observed (Blaxter 1986) that the visual acuity and reactive distance of larvae increase in relation to an increasing body size. An increase in body length also results in an increase in foraging efficiency. This is most likely related to morphological changes such as a larger gut capacity, an increased mouth size, and better maneuverability. Puvanendran and Brown (2002) speculated that it is not necessary to provide older G. morhua larvae that have passed the critical stages for development with very high light intensities. They noticed that the larvae reared in low light conditions had decreased mortality rates and insignificant differences in growth rates and feeding intensity in comparison to larvae in high light conditions. These results are particularly important in an aquaculture setting because a decrease in the amount of light required to grow larvae adequately means a decrease in the amount of energy required. Energy costs are a significant portion of the total costs of a hatchery, thus any decrease in these results in significant monetary savings for the organization.
Puvanendran and Brown (2002) also found similar results in regards to photoperiod; another significant variable which has affects on marine fish larvae. They observed the growth rates and survival of G. morhua larvae of different ages under various photoperiods. Three different daily lengths of time were chosen for photoperiod testing: 24 hours, 18 hours, and 12 hours. All rearing tanks were exposed to the same 1200 lx light intensity so as to minimize the amount of undesired variables. It was discovered that younger larvae reared in continuous light (24 hour photoperiod) were both longer and heavier than those raised in the other two photoperiods. A 12 hour photoperiod had the lowest growth rate, followed by an 18 hour photoperiod. This trend is also true for the survival of younger larvae. A 12 hour photoperiod was related to the lowest survival, followed by an 18 hour photoperiod. Larvae raised in continuous light had a significantly higher survival rate than both other photoperiods. The implication of this data is that the longer that G. morhua larvae are exposed to light in their early larval stages, the better they will grow and survive. During this study Puvanendran and Brown (2002) also observed the effect of photoperiod on larvae that had passed the critical growth period. They found that there was no significant difference between the growth rates and survival in the different photoperiods for larvae that were in this later stage. This suggests that continuous light may not be necessary during later larval stages. These results would also save energy and money if they were to be applied in a hatchery setting.
The reason that photoperiod has such a significant effect on younger larvae is thought to be related to the encounter rate between larvae and prey. The longer that larvae have the proper visual stimulation to be able to detect prey, the longer they have to forage and the greater their chances of encountering prey. This of course attributes to higher consumption rates, which correlates with better growth, nutrition, and survival (Puvanendran and Brown 2002). This is especially important to younger larvae because they are developing vital bodily systems, which need sufficient nutrition to be created and for the larvae to sustain existence. In addition, larvae are most vulnerable in these younger stages because not only are they smaller and have more limited movement, they do not have the proper functional developments to defend against or avoid most predators. An increased photoperiod is often associated with increased activity levels as well, which inadvertently affects growth through the amount of exercise the larvae performs. Increased activity increases muscle mass and thus size and weight as well.
It has also been observed that larval G. morhua will feed throughout the day given the opportunity. Suthers and Sundby (1996) found that G. morhua from the Arcto-Norwegian region had a higher growth rate than those of the Scotian Shelf region under the same photoperiod. This was speculated to have been due to the fact that Arcto-Norwegian larvae have more daylight hours in their natural habitat and thus have more opportunities to feed. With this in mind, the natural setting of an organism must be taken into account when determining the correct photoperiod for rearing that organism. Barahona-Fernades (1979) found that D. labrax larvae had the highest growth not in continuous light, but with an 18 hour photoperiod. This resembles the photoperiod in the natural habitat of D. labrax when exogenous feeding begins. Continuous light was found to actually impair the development of D. labrax. Similar results were also found during studies with Mylio macrocephalus (black porgy). It was discovered that under a 13 hour photoperiod there was a higher survival rate than at extended photoperiods for this species (Kiyono and Hirano 1981). Though as with G. morhua, it was found that at first-feeding stage Siganus guttatus (rabbitfish) also survive better under continuous photoperiods (Duray and Kohno 1988).
An important side note to consider is that in nature, photoperiod and temperature are generally interrelated (Boeuf and Le Bail 1999). This means that photoperiod may not be the only variable affecting feeding, growth, and survival rates in nature. Temperature has a significant impact on the metabolic systems of fish, which could in turn have an effect on the feeding rates of larvae. Larvae that can metabolize faster can consume prey more often, thus having the ability to grow faster and obtain higher levels of nutrition. Though the specific effects of both of these variables combined have not been studied to my knowledge, it is another consideration for future studies to better understand the myriad of influences on larval fish development.
In conclusion, the effects of photoperiod and light intensity on marine fish larvae are significant and widespread. These variables influence the growth, development, survival, activity, and feeding ecology of larval fish. The specific effects vary based on larval stage and species, and particular attention must be paid to this when preparing to rear a certain organism. Some organisms perform considerably better under higher light and longer photoperiods, while others suffer under the same conditions. Studies on the effects of light on larvae may have significant impact on the aquaculture community by allowing hatcheries to raise larvae faster and more efficiently. Though many organisms thrive in high light and extended photoperiods, oftentimes they perform with insignificant difference in more energy-efficient conditions. Knowing how light affects fish will allow hatcheries to raise healthier larvae that survive longer. Implications have been made that the early larval stages can be shortened through proper use of light as well (Puvanendran and Brown 2002). This could result in energy savings, which coincides with cost savings and increases efficiency. There are many other factors such as temperature, turbidity, and prey concentration, that combined with light have alternate or similar effects on larvae, and it is important to note this when doing research. Future studies may want to focus on the combined effects of temperature and photoperiod, or perhaps how various light spectrums influence larvae. An increased knowledge of the effects of light on marine fish larvae can only be beneficial.
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