Fish Schooled: fish

Showing posts with label fish. Show all posts

Tuesday, November 23, 2010

Postdoctoral Research Begins - Finding Sites

In the last week I have begun to work on my new postdoctoral research that focuses on how elevated turbidity levels can impact on exotic and native fish populations. This work forms part of a grant that I was awarded earlier in the year, and will continue to fund me for the next few years.

I arrived back in New Zealand this last Saturday and have been looking at sites to initiate field sampling. I will be working in the Waikato River in New Zealand. The Waikato River is the longest river in New Zealand running for 425 km in the North Island. It has a strong turbidity gradient that I aim to sample fish biodiversity along. Some site searching managed to turn up some turbid sites and some clear water sites. Here are some pics.

Whangaparino River Turbid Site

Waikato River Turbid Site

Waikato River Clear Site

Koi Carp Feeding In Turbid Waikato Tributary

Tuesday, May 11, 2010

Senses of the Deep Sea

The most abundant life on this planet is found deep beneath the waves at depths that sunlight hardly penetrates. Species that live at these depths are impossible to capture for behavioural studies where questions can then be asked about how fish at these depths 'see' the world. Thus, deep sea scientists are restricted to undertaking morphological analysis on these organisms and interpreting the results. One of the long held beliefs is that vision becomes less important for fish the deeper you go, and this is obvious when you look at eye size. Generally, after a certain point, when sunlight can no longer penetrate, the size of fish eyes becomes increasingly smaller and the non-visual senses become elaborate and highly specialised. In this post I discuss two papers that investigate what senses are utilised by mesopelagic and abyssal demersal fish species. For those of you who do not know mesopelagic fish are those that swim in the water column at depths of 500 and 1500m while abyssal demersal fish are those found near the bottom of the sea floor at depths of 2000-6000m.

What the author did was capture fish during deep water trawls in the Atlantic and Pacific oceans, and investigate their brain morphology. He described the brains with special reference to the differentiation of the sensory centres - olfactory bulb (smell), optic tectum (vision), octavolateral region (water motion via lateral line), and gustatory lobes (taste). For those who do not know what the lateral line is it is a hair cell based sensory system that detects local water movements surrounding the fish. Such as the wake of a passing prey. By comparing the size of each brain region for a specific species, with the overall average across all species, the author was able to determine whether the fish was a specialist for that particular sensory system.

Some species were 'specialised' in one particular sensory system (mesopelagic fishes 36%; abyssal demersal fishes 40%). Other species 'dominated' in two sensory systems (mesopelagic fishes 49%; abyssal demersal fishes 46%), while the remaining species were generalists and specialised in three senses (mesopelagic fishes 15%; abyssal demersal fishes 14%). No fish therefore were not specialised in any sense.

For mesopelagic fishes that were specialists 92% were masters of vision. This pattern, although not as strong, continues to hold true when you take into account 'dominated' and 'generalist' species. Sixty one percent of fish had above average volumes of the optic tectum. This would suggest that vision is the most important sense in the mesopelagic environment. This pattern is not as strong for abyssal demersal fish with fish specialising in vision in only 50% of cases both for specialists and when dominated and generalist species are accounted for. Vision therefore seems to play a lesser role in the deeper abyss.

The use of the lateral line shows the reverse trend becoming more important the deeper you go. Mesopelagic fish that specialised in the lateral line (25%) were less than those found in the abyss (49%). However, in both environments the lateral line was the second most important sensory system. This pattern of increasing importance of non-visual senses in deeper waters continues to hold for the other senses also. When shifting from the mesopelagic to the abyss, species with above average gustation areas (taste) increased from 10% to 34% , and from 3% to 37% for olfaction.

This pattern shows that the sensory environment of the abyss is markedly different than the open waters of the deep sea. Vision was clearly dominant in the mesopelagic waters, and this may be due to the abundant sources of bioluminescence found in this environment. This is quite evident when comparing the species between the two depth categories. Bioluminescent species were common in the mesopelagic, but not a single bioluminescent species was found in the abyss. With a lack of any visual cues for feeding or mating it is not surprising that non-visual senses begin to become more dominant at greater depth.

Wagner, H. (2001). Sensory Brain Areas in Mesopelagic Fishes Brain, Behavior and Evolution, 57 (3), 117-133 DOI: 10.1159/000047231
Wagner, H. (2001). Brain Areas in Abyssal Demersal Fishes Brain, Behavior and Evolution, 57 (6), 301-316 DOI: 10.1159/000047249

Thursday, April 15, 2010

Prey populations explode as predators get smaller.

When top predators are removed from ecosystems their prey and/or competitors increase due to decreased predation and competitive release. However, can changes in behaviour, or body size, of the predators also cause this effect? If true, this would be most evident in heavily exploited marine ecosystems where size selective fishing has lead to rapid reductions in the size of top predators. The authors in this study used a 38 year time series to examine the relationship between predator size and prey biomass within such an ecosystem, the Western Scotian Shelf.

Their analysis showed that since the mid 1990's predator biomass has remained relatively constant. If one species of predatory fish was overfished it tended to be replaced by another species of predatory fish. Yet, despite no changes in predator biomass, prey biomass has increased by a huge 300%. Statistically, what matched this increase most closely was a decrease in the size and body mass of fish at higher trophic levels. The mean lengths of benthivores decreased by 21%, piscivores by 8%, and planktivores by 16%. When translated into body mass large benthivores decreased by 59%, medium benthivores by 48%, piscivores by 45%, and planktivores by 34%. For example, a haddock in the 1970's weighed, on average, 2 kg, but now weighs approximately 0.8 kg.

The empirical results from this study support the hypothesis that reduction of predatory fish size is the dominant factor in the underlying explosion of prey biomass. Why would this occur? Larger predators have been shown to be more successful at capturing prey due to their faster swimming speeds, and greater visual acuity. Thus, larger predators can consume more prey per unit time than smaller predators, and as a result larger predators can regulate their prey populations more effectively. As predators get smaller, a reduction in predation pressure results, leading to large increases in prey populations such as the pattern observed in this study.

Shackell, N., Frank, K., Fisher, J., Petrie, B., & Leggett, W. (2009). Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem Proceedings of the Royal Society B: Biological Sciences, 277 (1686), 1353-1360 DOI: 10.1098/rspb.2009.1020

Tuesday, April 6, 2010

The Top Down Effect Of Turbidity Within Marine Ecosystems

Most studies on turbidity investigate freshwater ecosystems and few studies have focused on the impacts of turbidity on marine ecosystems. Eianne et al. (1999) showed that invertebrate planktivores (jellyfish) replaced planktivorous fish within Norwegian turbid fiords. This was likely to be because increased turbidity levels reduced the possibility of foraging in visually oriented fish, while tactile feeding in jellyfish allowed them to continue to feed under light-limited conditions. A reduction in fish populations was unlikely to be a result of a reduction in plankton abundance. In fiords where fish populations were reduced zooplankton were more numerous and grew to larger sizes. This confirms modeling and experimental studies which show that turbidity is likely to have a top down effect within marine ecosystems by reducing the ability of fish to feed visually and this in turn leads to changes in prey composition.

Eiane, K., Aksnes, D.L., Bagoien, E., & Kaartvedt, S. (1999). Fish or jellies - a question of visibility? Limnology and Oceangraphy, 44 (5), 1352-1357

Wednesday, February 17, 2010

How To Assess Fish Populations?

Source: AIMS - Australia's Tropical Marine Research Agency

Studying fish populations has long been the domain of underwater visual census surveys (UVC). Using this approach is useful in that the technique is straight forward and inexpensive, as it usually involves SCUBA divers or snorkelers counting fish along a transect. This method is a critical part for monitoring environmental changes over time or conservation efforts. The problems with such a method is that it involves a human observer bias, which directly or indirectly affects fish behaviour. Fish tend to be scared off or attracted to the noise and sight of the observers (Willis et al. 2000; Radford et al. 2005). Thus, it has been proposed that such methods bias any results during fish quantification.

Recently, baited underwater video (BUV) has been utilised to assess fish populations and it was found to be far more successful at estimating fish density (Willis et al. 2000; Dearden et al. 2009). This method relies on a camera that typically sits directly above some smelly bait in a container, and records any fish that is attracted to the bait. The advantages of such methods is that the human observer presence effect is eliminated.

Source: Tepapa NZ

However, a recent study comparing BUV with UVC has found the opposite result (Stobart, 2007) . Here is the abstract:

A baited underwater video (BUV) system for the study of reef-associated fish populations on shallow (10–20 m) rocky habitats in the western Mediterranean was assessed at four locations in Spain and two in France. We describe the apparatus and optimal deployment times for video sampling. Different species had different response times to the bait, with four response groups identified. Examination of species accumulation curves and fish abundance estimates over time revealed that a period of approximately 20 min deployment was sufficient to capture most species on video. The technique sampled a wide variety of species, with 51 species belonging to 33 families recorded. Nine species of fish appeared regularly at the bait in relatively high numbers, and consist of six carnivores (Serranus cabrilla, Serranus scriba, Coris julis, Diplodus annularis, Diplodus vulgaris, Thalassoma pavo), two planktivores (Chromis chromis, Boops boops) and one omnivore (Oblada melanura). However, abundance estimates for other species were generally very low (mean b1 per location). Comparison of results from BUV with those obtained by Underwater Visual Census (UVC) at the same locations suggests that although BUV estimates species richness reliably, UVC is the more suitable technique for estimating the abundance of shallow-water reef fish in the Mediterranean. BUV improvements are suggested to optimise its use in deeper waters where UVC using scuba is inoperable.

The author also correctly states that the amount of effort required to undertake a BUV based estimation of fish abundance may not be worth the effort. This requires hundreds of hours (perhaps thousands) of painstakingly slow video analysis. It is also only feasible to have one BUV apparatus deployed at any given time unless you have a large vessel, which would be expensive. Thus, the field work component would also take a much longer time than using SCUBA divers. The advantages of BUV is that it is not biased by human observers (although training can minimise this problem), and data can be continuously checked after the event.

In the end I think it largely comes down to the geographic locality and the fish species involved. Different fish species behave differently, and therefore one method might be better suited to that particular type of fish. For example, nocturnal fish are likely to be deterred by the bright lights produced from torches at night, and may be more attracted to bait due to relying more heavily on non-visual senses such as olfaction. Thus, for nocturnal fish the BUV maybe a more appropriate methodology. Also, no technique is perfect and each would have it's own advantages and disadvantages. As the authors suggest it would be most beneficial to combine the methods so that they are able to cross check one another. Unfortunately, this is likely to not be feasible the majority of the time.

Willis T, Millar R, Babcock R (2000) Detection of the spatial variability in relative density of fishes: comparison of visual census, angling, and baited underwater video. Marine Ecology Progress Series 198: 249-260

Radford C, Jeffs A, Tindle C, Cole R, Montgomery J (2005) Bubbled waters: The noise generated by underwater breathing apparatus. Marine and Freshwater Behaviour and Physiology 38: 313-356

Dearden P, Theberge M, Yasne M (2009) Using underwater cameras to assess the effects of snorkeler and SCUBA diver presence on coral reef fish abundance, family richness, and species composition. Environmental Monitoring and Assessment

Stobart et al. (2007) A baited underwater video technique to assess shallow-water Mediterranean fish assemblages: Methodological evaluation. Journal of Experimental Marine Biology and Ecology 345: 158-174