When do tadpoles emerge?
I have three small ponds and have had many varieties of toads, frogs and treefrogs over the years. All spring I have seen and heard MANY varieties and quantities of toads and frogs. Choruses every evening and all night. Every year, I catch hundreds of tadpoles a day to feed to my red eared slider in a separate pond. It is now 6/2 and I have seen ZERO tadpoles in the three ponds. I do have small 3-4" koi and goldfish in the ponds (always in there). Do they eat them (never have before - they get koi food every day)? Where are the tadpoles?
Water snakes will eat tadpoles. Raccoons also eat tadpoles (at least in North America they do), as do such predatory birds as great blue herons. Fish such as bass and carp would swallow them by the bucketful if they can. (Koi are a kind of carp. The bigger they are, the more they will eat.)
However, you do not mention that you had tadpoles that subsequently disappeared, but rather the tadpoles never appeared, correct? This can be caused by weather conditions, specifically harsh freezing temperatures that kill egg masses before they hatch. Of course, toxic chemicals can kill them, too.
Nothing has changed in my ponds (2 small 350-500 gal) 12 and 20 yr old . Very healthy plant life, very healthy small goldfish 2-6" - 6-10 in each pond, well aerated. This winter was not harsh - all plants and fish survived. Every year these ponds are teaming with tadpoles of all sizes/species from April -June. Toads, frogs and treefrogs were very active this spring (see and hear them day and night, I sleep with window open to listen to them). Never saw any eggs, and have not seen one single tadpole. Normal years, I scoop out 1-200 tadpoles twice a week or more to put in my red eared sliders tank. This year nada.
We have not had reports of issues with tadpoles and we do not have research on the subject. In nature populations ebb and flow. Some areas had a hard freeze after eggs were laid depending upon the microclimate. Hopefully, you will have tadpoles next year.