Author: John Turnbull
Marine scientists use a range of methods to study the underwater world. Each method has strengths and weaknesses, and there is no one method that can be considered to be "best" or "ideal". For example, one of the most common methods in recent times, UVC or Underwater Visual Census (divers counting fish) has been shown to underestimate true fish numbers by between 30 and 70%. Nonetheless, non-intrusive methods such as UVC are preferred by many researchers compared to more destructive methods such as trawling or poisoning using chemicals, for obvious environmental reasons.
So what are the specific methods used by researchers, in what circumstances, and why? All good research starts with observations, such as “there seem to be more tropical species finding their way to Sydney these days”. Observations then lead to a hypothesis, such as "we will find an increasing proportion of tropical fish species in Sydney each year in summer, but we won’t find them in winter as it’s too cold". There might be further hypotheses, such as "we will find less tropical species as we move further south". Hypotheses then guide the design for the study; in our example above, the study would need to count fish of various species, specifically separating tropical from nontropical species, at various times during the year and over multiple years. We might choose to count fish at Shelley Beach, another site up the coast such as Port Stephens and a third site down the coast such as Merimbula to see if there is a tropical vagrant “gradient” down the coast.
We might be interested in several variables as well. One of the most common is abundance – how many of each species is there? Other variables include biomass (weight of organisms), diversity (the range of species), life stage information such as size of individuals and behaviours such as what are the fish eating? We may choose to just study what is there as-is, or study the impact of a change such as caging off an area to exclude herbivores. If we choose the latter we need to make sure we also study an area that we have not affected, known as a “control”, to pick up any unintended influences.
Within each survey site we want to take several repeat samples, which we can average out later to account for the inherent patchiness of underwater habitats. If we have controls, we also do multiple samples in these. This means that with three locations, each with perhaps four sites and four transects, over three years sampling every two months we will have 3x4x4x3x6=864 samples to do – plenty of diving! Once it’s finally time to count stuff – how do we do it?
Transects are one of the most common sampling methods, especially for fish census. We lay out a tape and swim along it over a fixed distance, either counting as we go or taking video or photos. Video/photos are good as they give us a permanent record for later analysis, but on the downside it’s often hard to identify species from the footage. Transects often require two swims; in one direction we count all the species in the water column then on the return swim we count all the species on the bottom amongst the rocks, seaweed and coral. Belt transects are of a fixed width (eg 2 m either side of the tape). Linear transects measure the proportion of cover of each category, for example algae verses animals and bare rock, lying directly underneath the tape.
Roving Diver Technique involves a diver swimming slowly for a fixed duration, say 30 mins, recording species sighted along the way. RDT routes can be linear (swim towards a point) or random / circular. We use this method to survey species that are cryptic (hard to find), in low densities, are scattered over a large area or are in shallow habitats as the line and belt transect methods are limited in these situations. For example, RDT allows us to seek out species such as lobsters and black rock cod in their caves.
Quadrats are squares of a fixed size that we place on the bottom, then count or photograph everything inside the square. They are no good for many fish of course, but are preferred for sessile or slow moving species such as invertebrates and seaweed. They can be used for territorial fish that stay in one place, such as damselfish. Quadrats can also be divided up into grids to allow accurate estimates of size or to estimate percent cover of seaweed.
Stationary visual methods involve a diver sinking to a certain depth or to the bottom, counting in one place as the fish come and go. Set time intervals, fixed frames through which fish pass and fixed radial distances can all be used to standardise this method. Stationary visual is good for measuring size and abundance but is biased towards fish that swim about.
Manta tows and diver propulsion vehicles are used to cover large distances; around 1 km in 30 mins. With manta tows, a diver holds on to a board that is towed behind a boat. It’s hard to write notes as we go, so these are broad brush methods that are typically used for general qualitative assessments. They are also good for video collection. Not bad work if you can get it!
Remote video is increasingly popular in two forms; baited video and remote operated vehicle (ROV). In baited videos, we leave a video camera in a location pointing at a food supply such as fish heads in a netting bag or seaweed tied to a rig. The video then captures the fish that come to the bait. ROVs are small submersibles, typically controlled from the surface via cable, with video cameras attached. They are useful for studies below 30-40 m where diving is limited, or where there is a chance that divers may disturb the behaviour being studied.
Grabs and corers are used to take samples of soft sediments in what is effectively a three-dimensional quadrat. This allows us to study the organisms that live in the sediment back on shore or on the boat. This is an under-studied area and researchers are finding surprisingly high levels of organisms in marine sediments.
Marking and tagging allow us to study individual organisms over time; something that is important to understand what organisms do such as where they go, how they grow, when they reproduce and how long they live. Whilst these methods are temporarily intrusive, they are essential in order to understand the life cycles of species.
Traps, nets, trawls, set lines, anaesthetics and baited stations can be destructive or non-destructive and may be necessary to allow tagging, analysis of diet through stomach contents or age through ear bone patterns. They can be left for a period of time, which is an advantage for highly mobile species such as sharks.
Once the data is collected, statistical analysis techniques such as ANOVA (analysis of variance) are used to determine our reportable results. For example, if we have found 12 more tropical species this year verses last year, is the difference significant or just random? After all, fish populations move around and go up and down all the time.
We calculate the probability that the variance (+12 species) may be random and consider the result significant if this probability is less that 5%. The results are peer-reviewed and then published in academic journals and reports. As “citizen scientist” divers, we can contribute to research in a number of ways. We can volunteer to be part of programs like marine debris surveys or seagrass restoration. We can also find out the species that are of particular interest to researchers in our area and keep notes or photographs whenever we see them. For example, in Sydney, researchers are particularly interested in tropical species that are moving south due to climate change, grey nurse sharks and weedy seadragons to name a few. Every time we note and pass on information regarding such species we contribute in a small but important way to the understanding and management of our amazing marine ecosystems – and we get to have fun whilst doing it!