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New Zealand's Shallow Expanses

To many, it is curious that estuaries house large predatory fish, such as kingfish. Further, and happily, that the shallow nature of these systems permits the stalking of these fish by the angler on foot. In New Zealand, such environments are common; to however locate, intercept and catch such brute species therein is a different matter altogether. To better the angler’s odds, an understanding of the biological and hydrological processes that constitute such systems is crucial. These processes determine fish movements and consequently makes this article relevant to all anglers, inclusive of fly fishers, in all environments. I intend to discuss these everyday processes in our shallow water systems, since, if I may say, these are the most magical of places.

Estuarine environments are defined by the mixing of fresh and saline bodies of water. The consistent inflowing fresh and tidally fluctuating saline water presents a current rich environment. In deep time, the erosion, crafting and deposition of matter by these currents sculpted a barrier and so partly enclosing the system from open ocean forces. Uniquely, it is this shelter that permits an active nursery, breeding and feeding ground for inhabitants. Interestingly, the varying water, food, salinity and temperature force organisms to move about, with larger fish aiming to capitalise on this. The analytical fisher can deduce where fish, like kingfish, will ambush their unsuspecting prey by looking at where the most favourable environments are.

The large input of organic and inorganic matter from the land and ocean supports high rates of primary production by plants, algae and bacteria. This being the production of carbon-based compounds by the facilitation of the sun's energy – commonly known as photosynthesis. Carbon compounds (carbohydrates, fats, and proteins) being the universal building block for life; generally the greater the production of carbon compounds the more life a system can sustain. The organisms responsible for this inorganic to organic carbon synthesis are classed as primary producers and form the base of the food chain. The biomass of these producers determine the amount of life, and ultimately, the number of pelagics an ecosystem can sustain.

Phytoplankton, from the Greek meaning “plant wanderer” drift at the mercy of currents, and so also end up estuaries, and they are arguably the greatest primary producers of all. The most common phytoplankton members are diatoms, dinoflagellates and cyanobacteria. Their microscopic nature does not void their importance, as they are prolific and fuel oceanic and terrestrial ecosystems. By using light, the plankton synthesises not only discussed complex organic carbon compounds from simple inorganic states, but 80% of the world's oxygen too. This putting together of the organic compounds from simple molecules is unique to primary producers and separates the plankton from consumers. Consequently, it is the decaying, leaking and engulfing of the phytoplankton that allows the primary consumers to ingest the organic carbon compounds produced by the plankton and themselves grow, respire and reproduce. This process is how energy travels from the sun’s radiation to consumers, such as humans. The minuscule plankton run the world, and allow the world to run - not just your days fishing.

​​To manufacture such organic compounds, phytoplankton require nutrients other than the carbon. Nitrogen and phosphorous are upwelled from the ocean sediment by wind-driven currents, with iron provided by inland dust. In unison, these nutrients present conditions for plankton to reproduce, flourish and bloom. This being the main reason for many fishers having productive days after or during wind-driven periods, and if you haven't recognised that, it may be worth investigating. Furthermore, also why workups are near the coast or pinnacles where the upwelling is present.

Estuaries are distinctive in that the inflowing body of water consistently provides the essential carbon, nitrogen, phosphorus and Iron. Alongside the phytoplankton who enter at the greatest concentration with the incoming tide, an abnormally productive and lively system is formed. This being a productive period for the fisher, and consumers to target their quarry as the estuary further livens.

The cockle Austrovenus stutchburyi needs mentioning as it is a dominant consumer in New Zealand's estuarine systems. The cockle resides briefly below the sand and by the use of an in and excurrent siphon, it sieves through water for organic matter and the mentioned phytoplankton. It is unfathomable that this estuarine cockle population may sieve through a third of the incoming tidal water. Each shallow expanse is proportional to the mass of cockles it sustains, and so the volume of water filtered, resulting in a relatively constant estimate. This primary consumer is a food source for many organisms, specifically fish, while also a delicacy for humans. Further, the quality and clarity of the water is largely due to the cockles’ filtering ability and so they are economically, ecologically and culturally important - and permit the fisher to sight the query in what would otherwise be a sediment dominant area.

Moreover, the plankton is grazed upon by other primary consumers, such as krill, zooplankton and baitfish. Being a food source for many, the plankton’s life is short-lived; however, their minuscule size allows them to reproduce on an hourly basis in the presence of the mentioned essential nutrients and favourable conditions. Phytoplankton are not visible to the human eye, however secondary consumers, like cockles, are, and so their density is a good indication to the fisher of the primary production and health of the ecosystem - also, a very important component of the snapper’s diet.

For the shallow water angler, imitations take the form of these primary and secondary ​

​consumers. Most commonly flounders, mullet, jack mackerel, kahawai and piper: all are welcomed prey by the higher end predators. Often overlooked are crustacean species such as crabs, shrimps and krill. Although these primary consumers are smaller than many baitfish, when abundant and well presented they too are a meal. Having had kingfish and trevally refuse baitfish imitations only to devour a slowly retrieved shrimp, it is worth having in your arsenal. Generally, fish are inquisitive and feed on any matter likely to provide energy greater than what had to be expended acquiring it.

Seagrass, mangroves and salt marshes are also all primary producers, however not to the extent of phytoplankton. But they are equally important as they provide niches for many organisms, like juvenile fish, crustaceans and molluscs, to safely live. New Zealand's seagrass Zostera muelleri possess rhizomes - a root-like structure - linking each blade of grass and so stabilizing the bedding structure. This reduces erosion, allowing for habitats to form throughout a tidal flat. The upstanding leaves slow down water movement, trapping and depositing both organic matter and phytoplankton to again be harnessed by the primary consumers. Potentially the reason why great cockle populations are found in or around the mats of seagrass. Unlike phytoplankton, few species directly feed on seagrass but its importance in bedding stability, structure and oxygen production is unarguable. A NIWA study showed that the biomass of commercially and recreationally sought species like snapper, trevally and flounder are interlinked with the ​

​abundance of seagrass beds.

The fisher should seek to find dense beds of seagrasses, however, estuaries are not all equal in health, with many near urbanised, industrial and agricultural locations. These estuaries receive unnaturally high amounts of the mentioned nitrogen and phosphorus, and although these are essential constituents for the mentioned primary producers, like all things in life it is about balance. This nutrient influx permits other floral organisms, like the usually uncommon algae Ulva lactuca, to bloom. This alga is of great interest as it outcompetes the seagrass in the shallow light rich environment. The bright green sheets of the algae proliferate and proceed to smother the seagrass (among other organisms); the seagrass now unable to photosynthesise and grow. Consequently, the decrease in seagrasses biomass results in loss of oxygen production, bedding stability and refuge for many organisms. This vastly impacts your day on the water: not only as decomposing Ulva has a torturous smell, but by desertifying what should be a healthy environment too.

Like us, fish must breathe and acquire oxygen. Within our cells, oxygen allows chemical changes from nutrients to usable energy. In water, acquiring oxygen is not easy. In the same volume, the usable oxygen present in water is 2.5% that of the air, dependant on the temperature. The warmer the liquid the poorer the ability to hold dissolved oxygen, conversely, the cooler the water the more oxygen-rich it is. As you guess, estuaries are unique, varying greatly in dissolved oxygen concentrations.

In liquids, oxygen may enter through the mixing of air or as a by-product of photosynthesis by primary producers. It is the mixing of the air most commonly by wind with the surface film that adds dissolved oxygen. Again, there is a correlation in the activity of fish and the wind, this time however it is not just the upwelling of nutrients but the presence of oxygen. Generally, the shallow nature in estuaries radiate heat and have a poor oxygen-carrying capacity, while the channels feeding the shallows are often cooler. A lack of oxygen limits the ability of the fish to perform necessary tasks results in hunting by fish often being too energy expensive. To best fish the shallows - I mean knee-deep water - often the fisher must be present in the early morning, before the water warms. However, there are further variables to incorporate as cloud cover percentage, wind, tidal stage and season all impact and add to the puzzle of the temperature and may see that desired fish are encountered throughout the day.

Often not talked about, and linked with oxygen is the metabolic activity of fish. This being the mentioned chemical reactions within cells that produce energy. Unlike people, fish are ectotherms, meaning their body temperatures match closely their surroundings. If the temperature increases, the rate of chemical reactions too will increase and the fish will regularly need more energy. Interestingly, for each species the optimum metabolic temperature is unique. The varying temperature, and so oxygen in estuaries if analysed can be a great advantage to the fisher to depict fish movements.

Lastly, and arguably the greatest influence in fish movement throughout estuaries is the water salinity. The inflowing freshwater mixes with saltwater, consequently, salinities vary from .5ppt (freshwater) to 35ppt (saltwater); parts per thousand of dissolved salt, dependant on the tide size, tide stage and in flowing freshwater strength. This change in salt is minute, but fish cells are highly sensitive to it, with few able to function in both fresh and saltwater. Simply put, oceanic fish are composed of hypertonic cells, meaning that the salt concentration in the surrounding water is higher than internal cells and so water rushes outwards, consequently oceanic fish drink water constantly to survive.

New Zealand's shallow pelagics, such as kingfish, kahawai, trevally and snapper are briefly able to withstand and hunt in slightly reduced salinities. Consequently, for the fisher aiming to find these within estuaries, understanding where the saline water is located and how it spreads with the incoming tide is vital as this forms the margin of where many fish may hunt. An incoming tide fills the system with true saline water, pushing the inflowing freshwater back, at such point the harbour becomes filled with prey and predators. Conversely, as the tide retrieves, the freshwater advances through the estuary and disallows the hunting by many organisms.

For any fisher, understanding primary production and its relationship with food chains: oxygen concentrations and the relationship with temperature: and, salinity and how it varies with the mixing of fresh and saltwater will give you a good indication where the pack of meter long kingfish are hunting.

If there is one thing I wish you take away from this article, it should be that all things are intertwined directly and indirectly. Just as the butterfly may flap its wings in the Amazon and cause a tornado in Texas, so too may seagrass lead to the presence of kingfish. There are an infinite amount of variables present in the world, let alone our estuaries with only the major variables discussed in this article. I hope and encourage you to analyse your fishery as I have, and share interesting things you may find. I do not doubt that such a process will improve you as an angler.

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